MJ - I don't know a site to calculate CD vs Treasury with tax implications, but I was searching for the same info and found this on Zack's site:
To compare the interest rate from a CD with the rate from a Treasury bond and see if US Treasury bond prices and yields are a good deal, calculate the state-taxable-equivalent yield of the Treasury bond. The equivalent yield is determined by dividing the Treasury bond yield by one minus your marginal tax rate. As an example, say your state income tax rate is 8 percent and the Treasury bond you are looking at yields 3 percent. One minus 8 percent – 1 minus 0.08 in decimal form – gives 0.92. Divide the 3 percent by 0.92 to get a taxable equivalent yield of 3.26 percent. A CD must yield more than 3.26 percent to be a better deal than the Treasury bond.