I assume/hope you made a backup before flashing this update?
If so, I'd suggest you restore your backup and try again...if you didn't wipe your /System as a part of the install do it the second time.
If you did not make a backup before doing the update (a mistake I'll warrant you'll never make again, if you did

), and you are unable to boot up at all:
1. Load Clockwork Recovery:
- Boot into stock recovery by holding X while powering up
- Hit the magnifying glass on the keyboard to open recovery menu
- Highlight reboot and hit "ok" on the keyboard
- When you get the "M" screen pull the battery
- Power back up and it will go to Clockwork recovery
2. Do a backup of your current messed up state...this is a "just-in-case" what-the-heck backup, but having it might be useful at some point if problems persist.
3. Follow the instructions to install the 2.3.15 update, make sure you include the step to wipe system. Also wipe data/cache again before you install.
Boot up - it should come up fine and all your apps/settings should be restored. You'll have to setup your email accounts again.
Let us know what happens.