The IRS  has posted a schedule indicating when tax payers, if eligible, will receive their tax payments. Keep in mind that these are estimates and the IRS could very well not meet this schedule. The schedule is based on the final two digits of your Social Security number. If you file jointly, use the last two digits of the primary filer’s Social Security number.
If the IRS has your banking information on file, as they would if you entered the information on your 2007 tax return in anticipation of a refund via direct deposit, these are the dates by which you will (most likely) receive your refund.
00 through 20: May 2
21 through 75: May 9
76 through 99: May 16
If the IRS does not have your direct deposit information on file, then you can expect a check to arrive based on this schedule (approximately).
00 through 09: May 16
10 through 18: May 23
19 through 25: May 30
26 through 38: June 6
39 through 51: June 13
52 through 63: June 20
64 through 75: June 27
76 through 87: July 4
88 through 99: July 11
More information on the economic stimulus tax rebate at Consumerism Commentary:
- Economic Stimulus Tax Rebate Calculator (final version) 
- How Will the Economic Stimulus Tax Rebate Affect 2008 Tax Returns? 
Here are links to official IRS information regarding the new tax credit:
- IRS Announces Economic Stimulus Payment Schedules
- Economic Stimulus Payment Calculator (official version, but still an estimate)
- Economic Stimulus Payments Information Center