Bitcoin-Qt/bitcoind version 0.8.1 is now available from: http://sourceforge.net/projects/bitcoin/files/Bitcoin/bitcoin-0.8.1/
This is a maintenance release that adds a new network rule to avoid a chain-forking incompatibility with versions 0.7.2 and earlier.
Please report bugs using the issue tracker at github: https://github.com/bitcoin/bitcoin/issues
If you are running an older version, shut it down. Wait until it has completely shut down (which might take a few minutes for older versions), then run the installer (on Windows) or just copy over /Applications/Bitcoin-Qt (on Mac) or bitcoind/bitcoin-qt (on Linux).
If you are upgrading from version 0.7.2 or earlier, the first time you run 0.8.1 your blockchain files will be re-indexed, which will take anywhere from 30 minutes to several hours, depending on the speed of your machine.