No you will not get the rebate.
Also once you file joint return, you cannot amend it to file separate return.
If you do not receive full amount of $600 now, then you may get the balance amount when you file your 2008 return, and you become eligible for the tax rebate.
For information on stimulus tax rebate schedule, the eligibility and how much rebate you will get read:
Your U.S. Tax Return: Are You Eligible for 2008 Stimulus Tax Rebate Payment?