Senate Democrats caved to Republican demands and allowed the Tar Sands pipeline to remain in the budget bill. Obama had vowed to veto this, but has relented because the compromise doesn't force the construction to go ahead. The bill only requires Obama to decide the issue in 60 days instead of waiting until 2013. Republicans think they can't lose, Obama is damaged if he does or he doesn't. His safest move is to approve it, unless the liberal base makes enough of the right kind of noise about it, so that Obama benefits from killing it.
Update: Obama is now suggesting that putting a sixty day time limit on his deliberative process may force him to reject the pipeline.
Some pipeline facts: There are lots of pipelines although this is a large one, in itself the pipeline is not the problem. The company that is buying the oil has its refineries in a tax free "trade zone", the US will get no revenue, just the pollution. They are only going to make diesel fuel as the sour crude isn't good for anything else, this means very few refinery jobs and there is no need for the diesel in the US anyway. There is the small matter of burning four times as much Natural Gas equivalent to the oil being extracted to separate the oil from the sand. Why do this? There is no demand for more natural gas and it's available on site for practically free. It's like killing buffalo for the tongues.