My UK pension scheme (SIPP fund) owns commercial property in the United States. If my pension scheme sells this property, do I have to pay United States taxes on the transaction?

Your pension fund will be liable to tax absolutely.

This falls under the "Foreign Investors in Real Property Tax Act" or FIRPTA for short

See: Definitions of Terms and Procedures Unique to FIRPTA (http://www.irs.gov/businesses/small/international/article/0,,id=102266,00.html)