Bitcoin-qt v0.8.0b client on Mac OS X 10.8 crashes, why?
I fixed the problem myself.  During a system sleep or perhaps a hang and hard reset (graceless shutdown) the block index got corrupted.  Best-practices for Bitcoin-qt, at least on a Mac, are probably to quit out of the app before putting the system to sleep.

To fix the problem, I trashed the entire directory

~/Library/Application Support/Bitcoing/chainstate/

and then relaunched the client. It re-indexed all the existing blocks (took about 2 hours,) but did not have to re-download the entire blockchain.

After allowing it to fully re-sync and catch up all blocks, I was able to do transactions normally. 48 hours later the client is still running stably and normally. (Including quitting &amp; re-starting the client.)