Dani Rodrik, a prominent economist and sometime critic of free-trade conventional wisdom, backs the guest worker program. He doesn't get to my core worry, that we'll move to more European style immigration. However, having thought about it, I'm not sure how a guest worker that overstays their visa is any different than any current illegal immigrant. So long as any kids they have born in America are still American citizens, our model still holds. I would oppose any law that changes that.
Even more directly on point is this key fact (from TPM by Nathan Newman).
In the Bush 2007 budget, a grand total of $177 million was appropriated to enforce our wage and hour laws. Compare that to the $13 billion in the 2008 Bush budget for border enforcement -- nearly ONE HUNDRED TIME AS MUCH spent for border enforcement as for wage enforcement.
The comprehensive bill has provisions for upping employer side enforcement. Although, I suspect that under a Republican president those provisions aren't going to be pushed that hard. However, a Democratic President would be in the position to push hard on the employer enforcement side, so long as the fee-based amnesty was already in place. Labor would love it and, if done carefully, I don't think it would encounter strong opposition from immigrant-right groups. And, at this point the amount we're spending on enforcement is pretty minor, so substantially upping enforcement wouldn't be that expensive. If those provisions work, they should be fairly easy to defend and will force employers to lobby to raise immigration quotas if they want more workers.