Since you're already receiving checks, I'm going to assume you are over 59 1/2.
The withdrawal will be subject to regular federal and state income taxes at whatever bracket you're in. If possible, you should only withdraw what you absolutely need at this time to minimize the tax bite, or at least split it in 2 and withdraw 1/2 now and the other half in 2009. Also, since the market is down, the longer you can wait for it to recover, the better. The other consideration is that the addition of the IRA money to your other income may make more of your Social Security (if you're already collecting) subject to income tax. The addition of the withdrawal could also make your tax bracket change.
Please consult a tax professional in your area to help you with this. Make sure they're at least an Enrolled Agent and stay away from the the big "Block" and other franchises. Some know what they're doing, but far more don't!