What is happening
If you are using an old version of Bitcoin-Qt (or bitcoind, the server bitcoin software), you must either upgrade to version 0.8.0 or later before May 15, 2013, upgrade to an up-to-date "backport" release, or modify a file and restart bitcoin to work around a bug with the old software.
This bug does not affect any bitcoins you already have, but if you do nothing you will be out of sync with the rest of the Bitcoin network and will be unable to receive bitcoins (payments sent to you will look like they never get confirmed, or will be confirmed very slowly). You also risk being the victim of a "double-spend" attack, where somebody sends you bitcoins that the rest of the network would reject as invalid.
We recommend that you upgrade to version 0.8.1 before the 15th of May to avoid any issues. If you are a solo miner or mining pool operator, please see the the notes at the end of this page for how to upgrade safely.
If you cannot upgrade to version 0.8.1
If you cannot upgrade to the latest version, backports to older branches are now in final testing. These include not only compatibility for the 15 May changes, but also fixes for known vulnerabilities and bugs, which have already been included and tested in the latest release.
If you cannot upgrade to a backport
If you cannot upgrade to any of the above, you can still avoid the problem. Create a file called DB_CONFIG in the bitcoin data directory, containing this line:
Look here if you need help finding the bitcoin data directory.
Miners/mining pool operators
If you are creating new blocks (you are a solo miner or mining pool operator), then you should be aware that upgrading from 0.7 to 0.8 requires a lengthy re-indexing operation; you must wait for the reindex to complete before serving work to miners.
And if you are creating blocks and cannot upgrade to version 0.8.1 or a backport for some reason, you should not set_lk_max_locks in a DB_CONFIG file until May 15th; if you increase locks before then you run the risk of creating or building on blocks incompatible with the rest of the network.
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.