By the time we reach $250/BTC Difficulty will likely be around 18,000,000, so a 1 Ghash/s rig could still bring about $350/month.
This will be the case, unless the dollar price per bitcoin were to skyrocket in the coming weeks (from intense media exposure, I presume), before the difficulty even reaches 4,000,000. I guess we will just have to live and wait.