The Oregon Income Tax instruction booklet explains how the state recognizes your partnership for tax purposes.
Registered domestic partners (RDPs): To correctly
determine your Oregon tax liability, you must complete
a federal income tax return “as if” you were
married filing jointly or married filing separately. Use
the information you calculated on the “as if” federal
return to complete your Oregon income tax return,
unless otherwise indicated.
You have to figure out your federal taxes twice, one copy, the one where you file as single, you actually file to the IRS. The second copy (or as the Oregon Department of Revenue quite amusingly calls it, the "As if" copy) is the version where you get to pretend the federal government considers you a full citizen. You figure out how much you'd save by filing jointly, then use that information to fill out your state forms.
Domestic Partnerships are cool, I'm glad I'm in a place where there's that option. However taxes are just one of the instances where it becomes glaringly obvious that the whole idea is still a "separate but equal" compromise. That, and that handy list of "RDP's" that'll make it easier to round up the queers when the time comes. (Sorry, just a little conspiracy theory freakout.)
There's so many more issues that are so much more important to me, I really wish the polarizing push for "marriage" could just take a back seat for a little while while we fix the really big things, like equal legal protection for gays in that huge chunk of the country where one can still be fired or lose their apartment for being gay. Yeah, that would be a nice start...