There will be fewer than 50,000 U.S. troops in Iraq on May 21, 2010

Hard to call. As a science-trained individual, I see the situation as an equilibrium between cost and effectiveness. Add more troops, and you'll have to pay for them. Take troops away, and the remaining troops will have to face a higher amount of active "enthusiastic" resistance.

By 2010 I see the following things happening:
Obama withdraws a token amount of troops at the start, say 5-10%. Pressure to withdraw more. If they're intelligent, the resistance stays quiet. Obama withdraws more troops, say 30-40% of the original. Resistance goes active and troop deaths in Iraq climb. Media and opposition parties get into an uproar about "unsupported troops in Iraq". Troop numbers are either brought back up close to original number (with mild condemnation of Obama), higher (with widespread condemnation) or a complete withdraw (50/50 what this does to Obama's popularity.).

I see the Iraq situation as being too similar to Vietnam. During the entire Vietnam situation, politicians were entirely unable to bring active troop numbers down permanently from the (high) equilibrium position, until they brought the number of active troops down to zero.

50K ain't going to be enough for Iraq. Whoever gets elected will probably move the troops to Afghanistan. The strategy is always to keep enough military presence around Iranian force.

And no US president will want to give up control around that area. Any troop reduction is just PR to quiet down the crowd.

The pressure is on to reduce the troop. I say yes.

the number is always adjusted according to the latest development. it is impossible to predict until the place is secure.

I trust Obama will stick to his words, and reduce the troop count significantly. But first, we have to assume he gets elected. To me, the chance is 50/50. And I don't trust the government is willing to give up that much control in Iraq.