I think Ann answered the question the best.
I have been led to believe that because your girlfriend is on title she can claim the credit but you being the one holding the mortgage and depending how you file your taxes - could potentially pose a problem.
A good local source to get your question answered may be a title company - give Eden Toothman a call at Chicago Title - 510.527.2453
Remember also, you'll need to get your sale closed and recorded (to be safe) before the Thanksgiving holiday (register of deeds is closed that thurs and fri, for recording deeds). The $8K tax credit expires at midnight on DEC 1st... (the Tues after the holiday). So time is really getting short and lenders are letting us know that there is NO EXTENTION on the time and they know there will be a made rush! So, sooner is better.
The income limit for single taxpayers is $75,000; the limit is $150,000 for married taxpayers filing a joint return. The tax credit amount is reduced for buyers with a modified adjusted gross income (MAGI) of more than $75,000 for single taxpayers and $150,000 for married taxpayers filing a joint return. The phaseout range for the tax credit program is equal to $20,000. That is, the tax credit amount is reduced to zero for taxpayers with MAGI of more than $95,000 (single) or $170,000 (married) and is reduced proportionally for taxpayers with MAGIs between these amounts. (this is directly from the Federal Housing web site!!!!)
Always consult your accountant, your attorney and the IRS... everyone's in a different tax situations. HOPE THIS HELPS...
And don't forget to check with the IRS http://www.irs.gov/newsroom/article/0,,id=204671,00.html
When in doubt, go to the primary sources. Everything else is hearsay.