Is is possible to decrypt an encrypted Time Machine backup with Ubuntu Linux? If so, how?
Yes there is a way to mount a FileVault2 encrypted disk under Linux called libfvde, it uses the fuse Filesystem layer, but it is still experimental code.
The project is located here: http://code.google.com/p/libfvde/wiki/Mounting
If you are interested in the detailed analysis of FileVault2 which lead to this implementation look here: http://eprint.iacr.org/2012/374.pdf