I have an Adaptec AIC-7897 Ultra2 SCSI adapter on a system with 8G
of physical memory.  The adapter is using bounce buffers when DMA'ing
to memory >4G because of a bug in the aic7xxx driver. 

This has been fixed in both the aic7xxx and aic79xx drivers for some
time.  The problem is that these later revisions have not been integrated
into the mainline trees.

--
Justin