I'm talking about regular old run-of-the-mill DCS, the kind one gets from surfacing too quickly while diving. Has this been addressed in SR4 and I missed it? I don't recall whether or how it was addressed in earlier editions. Is it straight damage? A status ailment?

No rules for diving yet.

Nope, but I'd treat it as 3 possible outcomes:

Minor, like skin or joint pain. Will go away eventually assuming you don't do anything stupid, but might have a minus. Ranges up to a light CNS hit that doesn't get worse and gets better without treatment.

Major, a major or increasing CNS hit or chokes needing treatment to avoid death or crippling. Character is effective useless until multi-hour treatment is done.

Dead, AGE or a massive CNS hit. Don't do a panicked buoyant ascent from 150 feet. It's bad.

Actually, I was thinking to treat it as reverse falling. A fast ascent from 10m would be 7P damage. That 150' ascent would be like 25P.

The problem with that is that the length of time spent at depth is at least as important as the depth and speed at which you ascend.