Tough call. Brown hasn't even been flex-worthy with Vick at QB thus far, but once Ben is back he'll be the best receiver in the league again. You don't need Forsett with those other three backs, and Thomas hasn't exactly been world-beater this year with Peyton struggling. That said, Evans has been absolutely awful this year, so it really comes down to what your record is right now and whether you think you'll still be in playoff contention when Ben gets back.
You should also try to get something else back in that deal with Brown since, for the next couple weeks at least, you're giving up the two best players in the deal. Who else does the other guy have on his roster?