They'd apply to 2009, but as the other poster pointed out, since they would affect withholding, they would take effect immediately.
The real outrage here IMO is the business tax cuts. Under current IRS rules, businesses may carry a net operating loss back to the two years immediately preceding the current one, for the purpose of obtaining a tax refund on taxes previously paid.
Corporations use this loophole to "claw back" previously paid taxes from the government, when they lose money. However, the proposed change in "carry-back" tax losses (or carryover, I suppose) would now extend to five years, a huge bonanza for money losing banks, big homebuilders, and others.
This provision was specifically lobbied hard by the National Association of Homebuilders (NAHB) and other lobbyists. They tried to slip this in one of the stimulus packages last year, but were defeated by grass-roots organizers.
Please also check out this site, alleging that one of the major public homebuilders (Lennar) basically operated a Ponzi scheme a la Enron:
http://www.lenn-ron.com/Allowing these special interests to effectively loot the treasury to prop up their zombie companies would be a horrendous way to start the Obama administration. Please consider calling or writing your congress critter and tell them to please cut this out NOW.