15% of taxable income up to:
$69,000 (married filing jointly)
It's too late for 2011; the cutoff was 12/31.
When doing conversions this year consider converting assets whose values you believe have suffered temporary setbacks.
If you have a bundle to convert there's always the risk that the rules will be changed before you complete your conversions and the it will take longer for the tax free compounding in the ROTH to make up for the taxes paid on the conversions.