Google Co-op was just launched and, based from what I've read on other sites, most people really have no idea what the hell is it about. The Google itself is at fault because, frankly, their page about "Co-op" is rather confusing. Please read this very well written article with examples. Also look here.
Also, GTalk WWW interface is now supposed to have sound (embedded Flash should produce a beep when someone messages you). I don't hear anything. Is this a problem with Linux Firefox (maybe it requires Flash 8?) or is it simply not yet implemented?
UPDATE: Mystery solved! At least, the mystery of Flash beeping in GTalk. The trick is that it works (even under Linux) but the beep is only produced when the browser window with GMail/GTalk is not the topmost window! This is definitely a feature, not a bug.