Excellent question! The law allows congress to change the tax law for the current year! This means that on 12/31 congress can create a tax law that goes into effect on 1/1, 12 months prior. Of course, this can make tax planning difficult and congress is sensitive to that.
The hold up this year is the Tax Relief, Unemployment Insurance Reauthorization, and Job Creation Act of 2010 (the 2010 Tax Relief Act, P.L. 111–312 ). This is tax law that effects the tax year 2010! This is why we have to wait to file our itemized tax returns!