Any without a back (bumpers). Otherwise, the Speck PixelSkin HD will not mess with the flash.
If this is in reference to the available free cases given that they're all black you can use a black marker and it won't be as visible. Well unless you go with one of the clear options.
The clear options had/have the same issue. Alot of companies have rectified this issue and I would expect and hope that none of the free cases have an issue with the flash. Can't really complain because they're free though.