I contacted Yamaha regarding this issue and their technical support said that the RX-V2600 upscaling (not to be confused with upconverting) does not work with DVI, the options should be grayed out when connected in this manner. That's their official stance. He also said that there has not been a firmware release, and could not confirm whether or not one was in the works. Keep in mind that even if the UK has a firmware update available it wouldn't necessarily be a compatible fix with U.S. units since the UK uses PAL.
Now, obviously Clint's Westinghouse LCD worked, but might that be because it also has an HDMI input, which could make it more inclined to have the correct EDID information, or somehow else more capable of communicating with the 2600. I don't know for sure, but I do know that I'm not the only one having this problem.
If and when an update is available it would only be offered through an authorized Yamaha service center only, and don't expect Yamaha to let you know by way of an announcement. It's something you'll have to ask them about.