The newer AIC789x series Adaptec chips are supported under the CAM SCSI framework which made its debut in 3.0. Patches against 2.2-STABLE are in ftp://ftp.FreeBSD.org/pub/FreeBSD/development/cam/. A CAM-enhanced boot floppy is available at http://people.FreeBSD.org/~abial/cam-boot/. In both cases read the README before beginning.