5.1 has some big changes from 5.0, including the support for nearly double the number of processor architectures and a complete revamp of the way the kernel handles SMP (symmetric multiprocessing).
The article also details some changes to the FreeBSD jail system, that allows programs to be run in a restricted environment even when they must be run as root (think Apache, sendmail, named, etc.).
The 5.1 version is considered to be a technology release, which means that it isn't considered to be industrial strength (which means something in the FreeBSD world, my mail server is at 738 days and still going strong), but is instead there to test out the new technologies in preparation for a true 5.x Stable release later this fall. However, my experience has been that FreeBSD's technology releases are every bit as stable as most Linux stable releases.
And, of course, the best part is that FreeBSD is free, as in beer and speech, unlike Linux and it's GNU friends, which have a more onerous licensing model. All in all, this bodes well for the UNIX for people who want stability and security. (Oh yeah, did I mention that this is the basis for much of MacOS X?).
For those interested in seeing a very decent backgrounder on FreeBSD (and the other BSDs), check out this page from ExtremeTech. Although it leaves off MacOS X, it does touch on most of the major outgrowths from the work at Berkeley (and elsewhere, of course).