What should I do about the March 12th 2013 blockchain fork? Reddit/r/bitcoin is flooding with news that the Bitcoin blockchain has forked, in that versions 0.7 and 0.8 are accepting different blockchains. What does this mean for the users?
Gavin Andreson has tweeted this:
https://twitter.com/gavinandresen/status/311290936527298561

The bitcointalk post linked by Gavin is a post by Peter Wuille, which says this:

Hello everyone,

there is an emergency right now: the block chain has split between 0.7+earlier and 0.8 nodes. I'll explain the reasons in a minute, but this is what you need to know now:


After a discussion on #bitcoin-dev, it seems trying to get everyone on the old chain again is the least risky solution.
If you're a miner, please do not mine on 0.8 code. Stop, or switch back to 0.7. BTCGuild is switching to 0.7, so the old chain will get a majority hash rate soon.
If you're a merchant: please stop processing transactions until the chains converge.
If you're on 0.7 or older, the client will likely tell you that you need to upgrade. Do not follow this advise - the warning should go away as soon as the old chain catches up.
If you are not a merchant or a miner, don't worry.


An announcement has also been made at bitcoin.org.