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 [351]
Path:            /Applications/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt
Identifier:      com.yourcompany.Bitcoin-Qt
Version:         

 (

)
Code Type:       X86 (Native)
Parent Process:  launchd [107]
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