What kind of skill is Arcana?

It isn't a magic skill, since mundanes can have it.

Is it a Technical skill?

It is a magic skill. Magical skills require magical talent unless otherwise noted, not always.

Technically, I believe it is a magical skill, although I personally disagree with this, and would classify it as a Technical skill.

Whereas I personally would classify it as a Knowledge Skill.

There was a great deal of discussion regarding this. The best parallel I can give you is between a Doctor of Physics, an engineer, and a technician. The first one is all theory and conception, straight infodump - the equivalent of the Magic Background Knowledge Skill. The second takes that knowledge and does something with it; typically design or analysis - the equivalent of the Arcana Magical (but open to mundanes) Skill. Last, we have the guy that takes the engineer's design or instructions and actually uses it to build or accomplish something - the grunt labor, or in this case the equivalent of the Enchanting Skill, Conjuring Skill Group, or Sorcery Skill Group.

See, these really are three separate skill sets - pure theoretical knowledge, the knowledge of how to apply that to design or analyze, and the ability to turn those designs into reality. The reason Arcana wasn't lumped in with the Knowledge skills was to emphasize its "active use." A character with Arcana can synthesize and bring a new idea or solution into the world on demand; the Magic Background theorist has to wait for inspiration and genius to strike, while the poor Sorcerer/Conjurer is just miming things by rote with little clue as to why or his mojo works beyond the very basics.

"I say, that's the Golden Ouzon metavariant of the orangutan! They were supposed to have been hunted to extinction as virility aids by Japanese conglomerates. You know, it's said the..."
-Tommy Thaumaturgical Theorist

"Yes, I could go poking around in the ground looking for 'natural' orichalcum - or I could just finish designing this mageblade using a pint of gargoyle blood instead. Blood from the stone, neh?"
-Alistair Arcana, Focus Designer

"I put the mana in this end, and your face blows up."
- Sammy Sorcerer

So, for purely knowledge-based purposes then, would Arcana be pretty much interchangeable with Magic Background?

I wanna check my references, but in SR3 at least it was the case that every active magical skill had the equivalent of the specific Background skill built in - or at least that was the rationale for why a burnout's active skills were translated into such Background skills.

So yeah, you could rule Arcana as equivalent to Magic Background for most cases. The trade off is you can specialize a lot more in Knowledge skills - separate skills for any given subject to represent specialized knowledge in various disciplines and areas. Spirits, Conjuring Background, Sorcery Background, Parazoology, Parageology, Famous Magicians, Magical History, Unique Enchantments, etc.

That is all good, but it still doesn't clearly answer the question: Is Arcana considered a Magical skill or a Technical skill?

Street Magic seems to imply it is a Magical skill, however, as it deals with design, is linked to Logic instead of Magic, and is available to mundanes, it seems to fit quite well with the Technical skills (where I believe it belongs).

And finally, if it is indeed a Technical skill, why do the various Aspected qualities penalize it?

That's a good question, and I always felt like they shouldn't - if a mundane can do it without penalty, then someone whose magic is aspected shouldn't take a hit.

Personally, I like the idea of having it be a special kind of Technical skill.

It may be that some Aspects aren't as amenable to systematic exploitation of their powers as others. If your Aspect is influenced by or influences your thought process, perhaps it's harder for Magicians of some aspects to do the "design" work as has been suggested above.

That's great, and it makes a lot of sense, and you went above and beyond the call of duty to provide concrete examples that illustrate the point. But how does all this fit in with shamanism? I mean, as presented in Shadowrun, shamanism wouldn't really jive with the very logical system you just laid out. They wouldn't have "theorists," "engineers," OR "technicians" (or even equivalents of them). How does the Arcana model fit in with the intuitive nature of other traditions? I would be tempted to say that since shamans don't bother with thaumaturgical theories and formulae (in the nerdy, on-a-chalkboard, magicoscientific sense that always comes to my mind when I think of hermetics), they don't need to bother with other skills, but Arcana tests still rear their heads in the rules in places where I don't feel shamans should be at a disadvantage.

Sure they would - just because shamans don't emphasize logic doesn't mean they don't differentiate between lorekeepers, artists, and craftsmen.

A shamanic character with Magic Background will be adept at recognizing magical phenomena from many traditions, learned in the lore of different cultures and groups - they may not be able to design a spell to save their life, but they can recognize the scribblings of a mage or the rainbow serpent form a koradji's spirit of man might take.

Arcana represents the fundamental skill of evaluation and design, which are key and inherent parts of any magical tradition. An artist-shaman spends the same effort into designing her spells and foci as an hermetic mage, her composition just relies on a more intuitive (not to be mistaken with untrained or undisciplined) process and will appear differently - as a painting or a sculpture or a beaded necklace instead of a sheaf of paper or a datafile.

And the "technical" skills of Spellcasting, Summoning, etc. are dependent on the individual traditions but all involve the direct manipulation of mana - the methods might not be identical from tradition to tradition, but the essential process and results are identical.