Just wanted to tell you about my experiences.. even tho this doesn't answer you question. I don't have experience with autotest yet, so can't comment on that. This is Rails 3 with RSpec, DatabaseCleaner and FactoryGirl. Here are the results without and with spork:
Finished in 0.22748 seconds
3 examples, 0 failures
real 0m9.541s
user 0m8.733s
sys 0m0.772s
Finished in 0.24727 seconds
3 examples, 0 failures
real 0m0.605s
user 0m0.200s
sys 0m0.080s
So, from 9.541 seconds down to half a second. Roughly a ten fold performance boost.