We've now had two instances of this issue...
Following an upgrade to Yosemite, when logging in at the FileVault 2 login, you are then taken into the user account, with the progress bar. I believe this progress bar is unlocking the drive (What's displayed is the user's account name and profile picture, with a progress bar underneath it; looks the same as the Apple logo with the progress bar, but happens after authenticating with FileVault 2.)
The progress bar gets about 1/2 way through, then stops. We let one machine sit there for 8+ hours, it never moves.
The first machine this happened on, we eventually re-imaged and restored.
Then it happened again, and this time we think we found a work-around:
1) Boot into the recovery partition.
2) Verify and Repair the drive.
3) Decrypt the drive.
5) Login as the user
6) Re-encrypt the drive
At least you don't have to rebuild it from scratch.