BuMp! Same problem here!!! I can't figure out if it's connected, but if I started a tweet in Message Center - then click out of message center - and go back into message center - spinning ball and FREEZE! EVERY time. The only way to get it working again is a restart. I just noticed that Contacts is frozen as well.
Try some troubleshooting.
Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferences, the Guest account will not work.
If the problem is still there, try booting into the Safe Mode. Shut down the computer and then power it back up. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the grey Apple icon and a progress bar appear. The boot up is significantly slower than normal.