This is an interesting one - not so much TeamSite per se, but an issue we ran into on one of our TS Dev boxes.
We named our host client_ts.lab.ahundredanswers.com. This is a common naming convention for us, and worked fine til we had to worry about support in IE7.
For the life of us, we could NOT get the login to work. It would authenticate, load the top menus, and show login fields on the left and right nav panes. You could log in no matter how much you wanted and you'd get the same thing.
I dug around a bit and found that the browser was never setting a cookie. At first I thought maybe this might be some kind of incompatibility with the domains (client_ts.lab vs. client_ts.lab.ahundredanswers.com vs IP address), but no.
It turns out that IE rejects any cookies that contain an underscore. Our entire login issue was caused by the fact that we named our dev environment client_ts instead of client-ts or something similar.
Hopefully this will keep someone else from scratching their heads for two hours like I did today!
TL;DR: Never use an underscore in your TS environment names unless you never have to touch IE
