Please help! I thought I had created a new site before importing style settings, but was actually logged in to a site I worked on all day. Needless to say, the imported settings wiped out my site. Please tell me there's a historical log somewhere so I can go back to the previous version. I have looked but don't see anything. I will have lost about 10 hours worth of work today...

if this was a site updated from 3.4.6, the old settings from 3.4.6 will still be there.
go to ato->Export/Import Settings and hit the 'Delete bfa_ata4' button and it will rebuild it from what it last was in 3.4.6

If this is a new site and you don't have a backup of the database, you could try contacting your host to see if they have a backup otherwise I'm sorry to say that you are out of luck.

install the 'wp-dbmanager' plugin and BACKUP YOUR DATABASE on a regular basis