I personally think this is an idle threat—and I suspect W thinks that too.
I appreciate Rohrbacher's record on immigration—which unlike some doesn't include compromising on guest worker visas like H-1b.
I would humbly suggest though, that the time for threats is long past. Bush has lied to congress-and willfilly failed to enforce US immigration laws. It is now time for action. Frankly the immigration issue alone, would be grounds and there are others.
If Rohrabacher or other GOP congressmen introduce a formal bill of impeachment, it will be front page news. I would also suggest that the last time a GOP president was forced out of office by members of his own party, those that did so were able to bring one of their own—Ronald Reagan—into the White House a few years later. This may not be a perfect shot to bring immigration via the GOP-but I see no other at the moment. The question now is whether folks like Tancredo and Rohrabacher have the strength of will to seize this opportunity.