irs.gov has the answer:
Economic stimulus payments will be issued according to the last two-digits of the main filer's Social Security number. For joint filers, the payments will go out based on the person listed first on the return. Payments will be made by either direct deposit or paper check, consistent with how people filed their 2007 tax return.
People who use direct deposit also will be among the first to receive the payments starting April 28. Direct deposits will be made daily and completed by the date listed below:
DIRECT DEPOSIT
Last two SSN digits: Payments will be transmitted no later than: 00 through 20 May 2 21 through 75 May 9 76 through 99 May 16
Paper checks will also go out based on Social Security number. For Social Security numbers ending in 00 through 09, the paper checks will be mailed starting May 9 and will continue through May 16. A similar process will be repeated in the following weeks.
PAPER CHECK
Last two SSN digits: Payments will be mailed no later than: 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