The small amount of info I was able to find about the differences between each revisions are these:
A V7 will have motherboard GH-017, GH-018, GH-019 or GH-022 and markings on the BIOS in the range 070 through 080, A V8 will have motherboard GH-019 or GH-022 and BIOS 090.
There really isn't that much info and the meager amount of info there is seem to contradict each other (especially in terms of which motherboard and BIOS versions were featured on which revisions), so I am not even sure how accurate the info I found is.v8 seemed to be a test bed for some new anti DVD-r protection. They added some extra checks for DVD media on this model (which have already been cracked by two mods).
Bottom line is that new DVD game media checks were added on the v8 console, these are most deffinatly going to roll over to the v9 if not getting improved in the process. This is to protect sony from burned games since the console will read burned media so well now.