BitKeeper is a RevisionControlSystem? like CVS. From what I can gather it does things quite differently.

I believe it's quite different because CVS thinks about "per file" changes, where BitKeeper thinks about "changesets" -- PerryLorier

Linus is now using BitKeeper for the kernel sources since sometime around 2.4.13

More information here.

2002-10-10: There has been a large debate about this recently. BitKeeper isn't FreeSoftware, and they changed their LicenseAgreement? so that people could not use it who are working on a competing product, for example the GNU Project's SubVersion. There has (IMHO quite rightly) been a lot of anger over this move.