Can you have a look at your debug.log file?
You can find that in your home directory under Library/Application Support/Bitcoin
If you have no money in your bitcoin wallet, you can always try deleting that entire directory, so it can be re-initialized...
in my case, it's not even creating the Bitcoin directory, so the only error I can see is what Apple would get if you file a report:
I guess it's linked for 10.6 or later?
Process: Bitcoin-Qt 
Code Type: X86 (Native)
Parent Process: launchd 
Interval Since Last Report: 27077 sec
Crashes Since Last Report: 5
Per-App Interval Since Last Report: 0 sec
Per-App Crashes Since Last Report: 3
Date/Time: 2012-01-11 21:37:31.132 +0100
OS Version: Mac OS X 10.5.8 (9L31a)
Report Version: 6
Anonymous UUID: 8921A577-7713-4748-9665-8DCFF4C011AE
Exception Type: EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000002, 0x0000000000000000
Crashed Thread: 0
Dyld Error Message:
Symbol not found: __ZNKSt13bad_exception4whatEv
Referenced from: /Applications/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt
Expected in: /usr/lib/libstdc++.6.dylib