Take a look at my siggy. I have three bettas that are in rather large community tanks.
I have two that are in 10 gallons with livebearers, and they do just fine despite the very bright intense colors of the platies and the slight aggresiveness of the swords and mollies.
My other one is in a 25 gallon community, with everything from corys and clown loaches, to tetras and danios. I do have him in there with three serpae tetras and two zebra danios, and while I know that both species have a reputation as fin nippers, the seven of them get along quite well.
I also think that having the right kind of protection for the betta is very important too. I have a lot of plants in my 25 and both of my 10s so if the betta (or the other fish) need to get away from each other, they can do that.
But I would never put my bettas in with guppies or gouramis, just to be on the safe side.
Otherwise I think a betta in a community tank is a very good addition...if you have the right type of fishes and enough cover for them all.
...and thats just my two cents.