Coyote astutely points out:
There is legislation pending in Congress to restrict the ability of lenders (e.g. credit card issuers) from changing rates on existing debt. They ask if it is fair for someone who took on a debt thinking it would be at 15% to suddenly find it is at 25%. But how are tax increases any different. I make 10-20 year investments in my company, and the expected tax rate is a hugely important assumption in whether it makes any sense for me to put my capital in a particular venture. How is a large increase in taxes on returns from my past investments any different than changing the interest rate on an existing debt?