NEVER!
Just wait until 2012. Unless Obama is going down like Johnson in 1968, he will run unopposed in the Dem primary. That will leave every single Democrat available to switch parties to pick the weakest Republican possible. This could be very ugly, especially in open primary states where they don't have to declare their switch of parties a month beforehand.
The tight race between Clinton and Obama was the only thing that kept that from happening in 2008, but we still ended up with the Democrats' favorite Republican anyway. Imagine if they were actively working for it.