FreeBSD have early support for rapid STP in bridge(4): http://lists.freebsd.org/pipermail/freebsd-current/2006-October/066535.html http://people.freebsd.org/~thompsa/bridge_rstp.20061012.diff
I'll try and look at it sometime, but knowing how far I got last time I tried porting any kernel code (not very...and they have made quite a few changes to bridge(4) since importing it via NetBSD last year) I thought it may be worth drawing attention to here in case anyone else is interested.
