i looked on the IRS website and it said May, but heard on a local (SF Bay Area) news broadcast last week that "next week people can expect to see something in their mail box from the IRS... but it's not something to be concerned about but rather is the beginning of these fed tax rebates being distributed.
Additional Details
Judy and Richard - thx for your answers! they explain all aspects of my question. Posting here for others to cut to the chase and get the full picture without the need to sift through all answers:

From Judy:
The items you'll see before May are letters from the IRS explaining the program, and later a letter saying how much you can expect. Actual checks won't start going out until May

From Richard:
More than 130 million American households will begin receiving Internal Revenue Service letters next week reminding them to file a 2007 tax return in order to receive a 2008 economic stimulus payment.

The mailings by the IRS will begin the first week in March and continue throughout the month. The informational notice, titled Economic Stimulus Payment Notice, alerts people that they may be eligible for a one-time stimulus payment of up to $600 ($1,200 married filing jointly) starting May. There also is a $300 per child payment for qualifying children younger than 17.
Checks go out in May.