According to the rules, unbound spirits can be asked to perform a physical service.
What are people's ideas on limitations to this? In different writings from earlier source books and modules, spirits have been used for things like earth spirits diggging trenches or air elementals being used to fly characters. I was used and earth elemental to engulf my character and transport me through earth to a tunnel (something that I wouldn't let a pl;ayer character do in my campaign.)

Well I haven't had to limit this much, my players pretty much use Spirits to fight for them, open doors for them, or search.

so which spirits can open doors for people and how? Do they attack the door? Do they materialize on the other side and use the handle?
I usually have them attack my enemies (I'm both a gm and a player), but I've had players try to use them in a variety of ways.

If I had a fire spirit make sure a hellhound I owned didn't do anything bad by hovering over its muzzle in astral space until it tried to bite something or breath fire at it, would the babysitting as a whole count as one task, or would each instance of it coming out of astral space and covering its mouth?

As long as you tell the spirit to keep the hellhound from attacking people, and you don't phrase it to mean only once, then it will keep doing it as long as neccessary.

Some spirits are very literal, others are more freeform. Your choice of words ultimately decide it however. And the GM of course.

I heard that if you keep a spirit on a task too long, it'll get pissed off. If I have the fire spirit on doggie watch forever, when do I have to worry about this?

An unbound spirit has no fear of staying too long - only the higher force spirits given tedious, but simple jobs will ever get 'pissed' at you for it.

Bound spirits, on the other hand, can't go away if set to such a task. I would say you have aproximately a number of days equal to its force before it will have had enough and try to break free somehow...or at least set a vendetta against you...

Just a personal ruling judgement of mine. Of course, if you have treated this spirit well, given it treats, scratched behind the ears, etc, they will more than likely give you some mercy in time or forgiveness.

What would I give the spirit? Should I rent a bigass forge fire in an ironworking plant?

There's only so many things you can do to make a slave happy.

Bound spirits are stuck with you until you die, if you want them to be. They're on call 24/7, owe you however many services, and the only real way you have to "feed" them consists of sacrificing more binding materials and mana (read: taking drain) to them by Binding more, which, while it keeps the thing fed, also means it has to do even more for you than it did earlier. And there's always, always that threat looming in the back of the spirit's mind that if you felt the need, you could order it to drain itself out of existence to sustain a spell.

The initial summoning of an unbound spirit is a mutually agreeable contract, in a way... you've negotiated with the spirit (via the summoning test) for a number of services it will perform, in exchange for mana (which you take drain for channeling to the spirit). Obviously, higher force spirits will be better at this negotiation, owe you less services, and demand more mana, and that's pretty well rolled into the game system.

Binding, however, is a hostile act. It's the equivalent of shanghaiing the spirit... you summoned it for a short-term contract (until the next sunrise/set), and suddenly bang... you're trying to slap on the shackles and make it do things it normally wouldn't, and stick with you until you say it can go. Spirits are willing to help out with things like combat because they normally have little to fear beyond disruption... they get beat up and thrashed back to their home metaplane, which hurts a lot but they'll recover from it. On the other hand, when you've got it bound, you can order it to destroy itself, bleeding out it's life force over several days to sustain a spell for you until the spirit ceases to exist, on any plane. Suddenly, you're in a position to give an order that can get the spirit killed, and it didn't end up there voluntarily.

I'm sure the spirit would appreciate you giving it a nice place to stay while you've got it bound. It would appreciate not being your slave a lot more.

I unhderstand about hearth spirits opening dorrs to a degree but what if they're locked with a maglock?
And more importantly what about air spirits carrying you around? Seems like that eliminates any need for a levitate spell.

Maglocks still have ways of being opened without force - the door may very wel have a keypad or key swipe that your spirit can use if it 'liberates' the keys in some fashion, or you give them the data mentally if it can insert it themselves. In theory, a spirit could even go virtual if it has the right equipment hooked up. You'd have to teach them quite a bit to manage that trick though, or use analyze device as per the spell.

As per air spirits, yes they are quite fast for spirits, and the only thing faster is the good vehicles or a spirit/person under the influence of the movement power. Imagine one spirit of air augmented by another spirit with movement....wow! That's a lot of speed, hehe.