This article in Politico Has some additional details about the action surrounding Obama calling Brown.
In addition to the call to Brown, the article also says:

At a joint House and Senate leadership meeting late Tuesday, Senate Majority Leader Harry Reid promised to House leaders that he would put an immigration bill on the floor this year. House Speaker Nancy Pelosi, also in the Tuesday evening meeting, said the House would prioritize immigration reform, according to a Democratic leadership source with knowledge of the meeting.

IMHO, this is a trial balloon by the WH to see how much support/backlash there will be on the Hill and in the the community. Calling the opposition is a lot bigger deal than recording a video for advocates at a march. It reeks of real intentions to do things. Tomorrow it would be a good idea to call the WH and say thanks for making the call and urging that a bill be introduced in the Senate as soon as possible.

The number for the WH comment line is: 202-456-1111