Chat was just enabled on my Gmail account and I'll still very much checking it out. So far, I dig. I've really got to get to bed, but a few things first. The "Profile" box is unnecessarily spacious. Here's what it looks like:

Notice that big mass of white on the right? It may be there for a reason - I'll get to that in a moment - but for the time being, it is fat to burn. Here's an economic alternative:

(Obviously, a slightly modified layout is needed to elegantly handle long user names:

)
It is known that Google Talk is soon to add avatars, so that white space could be placeholder real estate for something like this:

But the issues posed by the aforementioned long user names would break this simple layout. I imagine these profile boxes will eventually contain pictures, but we must wait and see how things pan out. In the mean time, these could really be slimmed down. More quips to come, I'm certain.