That alone doesn't provide enough information to know which database file the error is occurring with. The debug.log might give a more precise indication.
If it is a problem with the blockchain, that is easily resolved. If the problem occurred after your computer crashed, that is likely what happened.
First though check to eliminate that the problem is that your hard drive partition is filled with no disk space remaining. That probably isn't the case on your Mac, but just in case, check first.
So, after making yet another backup up your wallet.dat, then you can do this to get a new copy of the blockchain:
These instructions are for mac. For Linux and Windows the directory names would be different.
$ cd ~/Library/Application Support
$ mv Bitcoin Bitcoin-bak
$ mkdir Bitcoin
$ cd Bitcoin
$ cp ../Bitcoin-bak/wallet.dat .
Then launch Bitcoin-Qt as normal. It will re-download the blockchain. This could take 24 hours or more, depending on a number of factors.
You might speed that up a bit by getting the blockchain as-of block 170,000 from here:
After it downloads, extract those files into your new ~/.bitcoin directory.
If after Bitcoin-qt downloads the blockchain to the most recent block and everything works fine (shows your wallet balance and transactions properly), you can then remove the .bitcoin-bak directory:
$ cd ~/Library/Application Support
$ rm -rf Bitcoin-bak
Also, the blockchain download is faster if you are on the most recent release of Bitcoin-Qt, v0.6.3.
wallet.datfile, make one now. If you do already have one, make another but don't overwrite the old one with this one. So long as you have some file that contains your private keys, you will ultimately be able to recover the coins.