If you’re running Bitcoin-qt or bitcoind, check to make sure that you have the latest version before May 15th. Running old versions after May 15th will cause you problems.

Bitcoin.org explains why this is necessary:

A bug caused a temporary block chain fork on 11 March, 2013. After investigating that bug, we determined that the bug can happen even if the entire network was still running old versions of Bitcoin-Qt/bitcoind. Therefore, the only option is to require everybody to either upgrade or workaround the bug.

You can get the latest version at bitcoin.org.