If you own an EVO and are currently switching between ROMs, you may have noticed a bit of an issue with its 4G connection. This has been found out to be a problem with the "RSA keys" that allow your WiMax radio to function properly. The brilliant users over at XDA have developed an application that will check your device to see if these keys are present or corrupted.
These RSA keys sometimes can get corrupted after flashing recoveries and applying ROMs and all you have to do is make sure they're in place to get your connection back.
XDA member joeykrim has developed a simple app that will determine if the RSA keys are still present or if they were wiped out in all your flashing fun. Getting them back isn't impossible, but it isn't simple, either - but the first step is to find out if the RSA keys are even corrupted or missing.
You can download the application, as well as get further information, in the XDA application thread.