Upgrading computers that matter to a new operating system version during the first week of release is considered a mistake by many.
Downgrading is a hassle and may be your only option.
Apple released a OSX 10.6.1 version in the last two days for testing.
Probably less than 10 per cent of the upgraders have seen problems.
Backing up your previous Boot Drive with SuperDuper! is free and easy, and allows reverting to the previous known good OSX very easy.
Paying $27 for the licensed version of SuperDuper! allows you to create a SandBox to test OS upgrades without danger.
Repairing permissions and repairing HD, deleting caches, verifying plists and clearing Virtual Memory BEFORE and AFTER an OSX upgrade is recommended by some. AppleJack will do this for OSX 10.5. Compatibility with 10.6 is a question still after a week.
ONYX repair utility is supposedly compatible, but I have not tested.
Snow Leopard FAQ at Macintouch.com
OSX 10.6 Compatiblity List at MacIntouch.com
There are reported FONT problems with Snow Leopard that can cause the app to crash during the Splash Screen Loading ... Screen.
I have no clear suggestions, Starting with the Shift Key down to force a Safe Boot with HD diagnosis and loading only Apple Extensions, Fonts and such may point the way.
Making a new User called TEST in Accounts in System Preferences, logging out and logging in as TEST may point to problems in an existing old account that are not present in a newly created user.
I run FMPA 10 and Snow Leopard. I've had no problems. I did maintain a SuperDuper backup of 10.5.8 until yesterday, then backed up my 10.6 System drive with the upgraded SuperDuper 2.6.1. This software has some serious problems. I've been in touch with Shirt Pocket a few times today and they've acknowledged a major problem related to !0.6 compression routines. SD is, however, safe to use and a real bacon-saver. The problem relates to "Smart Update" not being so "smart".
There is currently no version of Onyx compatible with 10.6.
I like AppleJack but don't know about a 10.6 version.
Upgrading to a new OS in the first week is NOT a mistake as long as you have a means to restore your system to its previous state.
Apple seeded 10.6.1 to selected developers. This is pretty standard behavior meant to iron out issues that come to light when thousands of users begin using a new OS simultaneously.