Hardly scientific, but there have been reports and photos that lead us to believe the white iPhone 4 is slightly thicker than the black model. Apple’s website lists the same specs for both models. They do mention the following which could account for the variances.
Actual size and weight vary by configuration and manufacturing process.
Plenty of opinions in the forum on this topic. If the white iPhone 4 is slightly thicker, I cannot imagine that case manufacturers will be a happy bunch.