I'd agree with the rest, it really depends on the betta and the fish it goes with. I've got one community tank with neons and a betta and they (mostly) get along fine - the very occasional bit of fin nipping but thats only for a few minutes or so every couple of months! I think the neons do something to piss him off!!
I've got another betta that can't go with anything - he's only young but he was nipping at corys three or four times his size! He doesn't like shrimp either, and attacks them (he doesn't eat them, just kills them), and he doesn't particularly like assassin snails either (likes to knock them off the glass) - he's been moved now, to a tank on his own where he's much happier!
My LFS keeps betta with guppies, and the results aren't good - having just bought a batch of guppy, out of 21 nearly all have fin rot and chunks out of their tails and fins (if only i'd checked properly before buying...!), and other than the guppy occasionally nipping each other I'd put the rest down to the betta they were with! needless to say, the guppy are all quite stressed and sick and are being treated as i write (sadly a few have died already, hopefully the rest will be ok!)
Personally I'd also not recommend keeping a betta with anything else - the one in the community tank I was very lucky with, and as nuttyJD says, it'd probably end in a bloodbath if I ever have to replace him! Having said that, I'm glad he's in there - the little community wouldn't be the same without him!
Good luck whatever you decide to do, but if you do put them together, keep a close eye on them all for missing bits of tail etc. and have somewhere you can isolate him if things go really bad!