I have tried a number of things and the VirtualBox Mac archive for 1.6.6 will not decompress.
If you try it in the GUI you'll get an endless loop between bz2 and cpgz files (double clicking will make a new copy in the other format). I also tried Stuffit Expander that halts at around 8 MBs and leaves an unmountable DMG file.
Further attempts from the command line with Bunzip2 yields a:
bunzip2: VirtualBox-1.6.6-OSX_x86.dmg.bz2: trailing garbage after EOF ignored
I tried using the FORCE option and ended up with an unmountable 35 MB DMG.
HELP!