If you're on Linux, make sure you're using Sun Java and not OpenJDK.
That's the only thing I can think of, apart from of course simply deleting your minecraft.jar and grabbing a new copy from
minecraft.net, like everyone else has said.
For the Sun Java:
sudo add-apt-repository "deb
http://archive.canonical.com/ lucid partner"
sudo apt-get update
sudo apt-get install sun-java6-jre sun-java6-bin sun-java6-jdk sun-java6-plugin sun-java6-fonts
--Darth