MacBook Pro freezes with beachball on login (OS 10.7 Lion)
Disable Automatic Graphics Switching

EDIT:  In diagnosing the issue, you should update to the latest 10.7.2 update

After browsing the Apple Discussions Forums more thoroughly, it seems that disabling automatic graphics switching in the energy/power settings of system preferences seems to prevent the freeze.  

This forces the MacBook Pro to use the discrete graphics chip which will consume more battery life, but you can manually switch to the other chip using the utility, GFXCardStatus (http://codykrieger.com/gfxCardStatus) This should make the computer usable until Apple issues a fix in 10.7.1 or whatever is next.

Unfortunately, users may have trouble just logging into their system to change this setting.  Typing in the password quickly (before the freeze starts) worked for me, but you may have to boot into safe mode instead (hold down shift on boot).