I'm in the process of filing for my wife's green card. She came to the US in Aug 08, now living with me, unemployed still. Would it matter whether file jointly or separately on my taxes for immigration purposes?
You will need to submit your taxes in connection with the Affidavit of support, and USCIS will review these and other financial documents for evidence that the two of you are sharing your lives financial as well as in other respects (in order to determine whether the marriage is "bona fide").
From an immigration perspective, it is far more helpful if the taxes are filed jointly. Speak to a tax professional concerning any tax consequences of this, and an immigration attorney for assistance with the filing.