"The kernel source level debugger, kgdb, has been around for a long time, but never in the mainline tree. Linus Torvalds is not much of a fan of debuggers in general and has always resisted the inclusion of kgdb. That looks like it might be changing somewhat, with the inclusion of kgdb in 2.6.26 now a distinct possibility.
"Over the years, Torvalds has made various pronouncements about debuggers, particularly kernel debuggers, a long message to linux-kernel in 2000 seems to outline his objections..."