Hardware-wise, the AT&T and DevEd phones are identical. The radio firmware is different, but I haven't noticed any particular difference going between the original AT&T radio (original stock firmware without any OTA's) and either of the two DevEd radio firmware's (original radio firmware and the radio firmware on the DevEd OTA.)
In terms of the warranty, HTC reserves the right to not honor the warranty for either of the model if the phone is modified. Being that the bootloader is already unlocked on the DevEd, that's not considered a modification. (On the AT&T device, unlocking the bootloader IS a modification.)
In actual practice, I've never seen a phone manufacturer void a hardware warranty for software modifications -- unless the manufacturer had good reason to believe that the software modification caused hardware to fail. (For example, a processor burns up from overclocking, screens that developed defects from being over-driven, etc.)