reduce corporate tax rates to 25%, but only for companies that increase their US workforce by 10%.

Controversy exists whether lowering US corporate tax rates will increase job creation. This proposal tests that premise without giving away anything in advance.
The IRS has all the Social Security filings from the last week of December, 2010. Take that as a baseline. If a company--measured by the same filings--adds 10% to its US workforce in 2011, it gets to pay taxes at a 25% rather than 35% rate. To prevent cheating, the filings would have to average a 10% increase over the entire year. Severe fines, and jail time, should be imposed on companies that cheat (e.g., inventing fictitious employees and just paying the social security taxes for them to qualify).
Instead of bickering about its effect, provide the certainty of lower tax rates in exchange for actually creating US jobs.