Have a look at the changelog:
http://svn.m0n0.ch/wall/branches/freebsd8/CHANGELOGRegarding NAT redirection - doesn't look too good, things haven't really changed with FreeBSD 8 as far as ipnat is concerned. We may eventually back down and put in a userland TCP proxy to provide at least a crude solution to the problem (IIRC pfSense did the same).