My friend has had about 40 sessions of acupuncture.He told me that it does help, but not that much.
Also i don't think that pelvic floor muscles have anything to do with prostatitis. judging from what it felt like, although that area was pretty relaxed to begin with, during the acupuncture sessions it felt like it was relaxing further. so maybe if those muscles had been the source of the problem i'd have seen better results.
It seems to be a surprisingly large body of evidence in favour of acupuncture - mainly from China, but there's also a bit from other countries. (interesting, one study i saw suggested the positive results of a similar size occurred in both :bacterial and nonbacterial prostatitis - pretty much contradicting what i said above. but the size of the effects in this study wasn't large)