This has been discussed here and elsewhere. For more complete info, please search the macosx and macnn forums before posting questions -- you'll find your answers a lot more quickly that way.
That said, Darwin 1.2.1 offers very little (if anything?) that OS X PB doesn't already have.
I don't know if the Darwin release number is anywhere in OS X. Darwin 1.2 was released after OS X PB, but was basically a catch-up to get the public Darwin synced back up to OS X... so maybe the version number wouldn't be anywhere in X? Anyone else know?