Our team is colocated, and we still chat all the time. It's a little odd. Basically, most of us have a chat window open on our desktops constantly, and we take a look at it when we're waiting (for compilation, a grep to finish, etc). It's that low-level background buzz. In addition we have, and frequently use, the option of simply walking over and talking to each other.
We tend to use chat for things that the entire group might be interested in, like:
- build failure notifications, and their causes
- discussion of which branch to run in the nightly automated tests
- notification when weekly lunch arrives (the important stuff!)
- code review requests
- heads up about interesting or risky checkins
- pairing requests
- general pleas for help (e.g, "how do I do X in perl?")
We get together from there. Someone will wander over and answer how to do X in perl, if it's non-trivial, or will start pairing. It generally works for most of us. I like it in particular because that way you don't have to feel bad about asking questions - you just ask the collective group and whoever is least concentrating at that point, or currently waiting for something, will answer. I'm not then interrupting someone who's really deep in something.
Use for yourself with caution!