True Crypt 5a was released on a few weeks ago on February the 12th, one of the major new features is the release of a Mac OS X version of this free encryption application. A quick test confirms that creating a TrueCrypt 5 encrypted volume as a FAT32 formated volume using the new OS X 10.5 Leopard release of produces an encrypted file that can be mounted and used on a Windows XP machine that also has the new version of true crypt installed or availible.
Personally I find this new version wonderful. My previous cross platform encryption solution was to use bcrypt a command line application availible for both Windows and Mac OS X, however this was really only useful for file encryption. Alternatively I could use an encrypted drive image in OS X, But then this was no use if i needed the files on my Windows machine.
True crypt 5 allows a user to set up an encrypted volume that can be mounted as a drive, in the same way as an OS X drive image, only now of course its cross platform, and if you run truecrypt in traveler mode, its portable too! Bonus!
In addition if you happen to have a Linux box, or maybe one of the shiny new Nokia Internet Tablets you 'should' also be able to run TrueCrypt 5 on those as well, TrueCrypt has supported Linux for a while I believe.
You can Download TrueCrypt here.