The distributed binary of bitcoin is compiled against libdb4.7++. However, compiling it against libdb4.8++ seems to work fine, at least in my tests on Debian. It seems that the FreeBSD people are also compiling against 4.8 (at least according to http://bitcointalk.org/index.php?topic=379.0
), and I think it would be a worthwhile pursuit to find out if others are able to use this newer format.
If you are willing to test, I'm interested in hearing what your results are. If you are, you should be *very careful* with your wallet, because hey that is your money. Backup your wallet before you mess around. If you don't know how to backup your wallet, you shouldn't be testing. Verify your wallet is backed up, if you don't know how to verify your wallet is backed up, you shouldn't be testing.
Now that you have verified your wallet is backed up, stop bitcoind, install libdb4.8++ and compile the bitcoin binary against it instead of 4.7. Make sure you aren't linking against 4.7 in the process. Then launch bitcoind and verify that things are working. The upgrade to 4.8 will automatically upgrade your database files to the new format and you should notice no difference in operation.
One issue with moving to 4.8 is that although going back to 4.7 is possible, it may be risky, so making sure you have a backup is important. To go back to 4.7, you just need to stop bitcoind and then delete .bitcoin/database/* and then start up the version compiled against 4.7.
Maybe you are using libdb4.8 already? If so, please post here which operatingsystem you are using.