White iPhone 3GS Discoloration Due to Third-Party Cases, Not Overheating?

Reports of overheating and discoloration on the new white iPhone 3GS have been circulating over the past week or so, but a new report [Google translation] from frenchiPhone claims that the discoloration is due to contact with some third-party cases and not associated with overheating. The discoloration reportedly can be removed by wiping the back of the iPhone with alcohol.