Operation Cast Lead was timed to end immediately before US President Barack Obama's inauguration. Since then, rockets have continued to be fired on Israel from Gaza, with limited retaliation, and preparations for a possible second round. Had Israel not pulled out, would that have put an automatic strain on Jerusalem-Washington relations?
I do think the Obama administration will be less friendly to Israel than the Bush administration. And I understand why the leadership in Israel might have wanted the operation finished by January 20. There may have been other reasons to stop, as well, although with the renewed launching of rockets, those reasons are less apparent.
Military operations like Cast Lead should be carried through to their own logical conclusions, and I think Israel has to calibrate its military actions based on its own self-interest. Trying to judge what it should do based on American politics is a perilous venture.