I have problems with this as well. Starting from the main menu, I can navigate to single player and subsequent menues just fine, but multiplayer locks up for a good 20-30 seconds between menues and sometimes freezes completely and stops responding. My computer is just fine, I run far more system intensive games online without problems. NVidia 8800 GTX, Quad Core processsor with dual sticks of 2gb ram, YES my drivers are updated.
This just happened to me actually after 3 warm up singleplayer games, i backed out, then went to go online and it took about 30 seconds to get to each multiplayer screen. Was fine yesterday and most the day today, but not now ):
Hopefully i can get back online in a few mins hehe