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
Source: By Consumerism Commentary blog