These are great suggestions--and good points about the limitations. Thanks! Do you think IRC is superior to other IM clients? A lot of folks in the offices have never used IM before--and may be more comfortable with something like AIM or GChat.
Skype has excellent group-chat, that's what we use. Much easier to adopt than IRC for non-tech types, and it's free which is a bonus.
(I didn't even know Skype had a chat feature, let alone group-chat, until I got to my current job.)
If you have people who haven't touched IM before, you should definitely checkout Campfire. One mass "company" room can get crazy so you can always fragment it off into a couple of area specific chats alongside the primary.