I'm assuming you're in the U.S. working on taxes due in April? If not disregard everything I say.
The taxes you are doing are for 2008. You file your return and figure out if you owe more or get a refund based on what you made/paid in 2008. Therefore anything you want to use as a deduction has to have been purchased between January 1, 2008 and December 31, 2008. Anything you bought in 2009 can be written off on next years taxes.
I'm sure that there are other exceptions, but the only one I know if is that you can contribute to a Roth IRA for 2008 anytime up to when you file your taxes.