Stickied. WP7/CE6/(future CE7) is quite the different ball game from WM2003/WM5/WM6/WM6.5/CE5/CE5.x...
There's also the LG Expo which runs QSD/ARMv7, so is a possible target.
The major hurdle with porting is that our current devices contain Radio ROMs, Drivers, and Bootloaders which all target the CE5.x platform. CE6+ is a different beast on a low level (in a good way, there are major improvements across the board which required breaking API compatibility - these improvements have been long overdue)
With access to whitepapers documenting the hardware's interface(s) and initialization routines, Platform Builder for WP7, and a good bit of time it's certainly possible to bring any platform supporting ARMv7 up to WP7. However such information/software is not public, so it's up to the OEM to either develop this (not at all likely) or release the requisite platform BSP (also not at all likely) to enable anyone to port their devices to WP7.
There are other possible methods such as emulation, but the amount of work involved would outweigh the end result by a good margin. The OEMs major concerns with porting ROMs is the sharing of 3rd party software to which they are not authorized to license distribution (for example, HTC licenses WM from MSFT, and also licenses a number of other technologies such as swype)
One possible solution for this in an ideal world is for Qualcomm to license HTC to distribute a binary-only driver solution, similar to Nvidias driver license on *nix. Combining that with HTC's own OEM specific binary-only libraries, would give us a bare-bones Platform BSP. This would contain only technologies owned by Qualcomm and HTC. They could then release this bare bones, binary-only, pre-linking BSP, and we could, on our end, combine it with the WP7 Platform Builder, and come out with a completely stock WP7 ROM.
This would allow us to constantly update the OS version, add our own 3rd party software as needed, and eliminate the need to use "cooking" tools to build a ROM (instead we could use MSFT's own PB or a derivative of such)
In this situation there wouldn't be any licensing issues outside of Qualcomm and HTC's own IP. Will they ever do this? I doubt it. But it would be very, very nice! It would satisfy the OEM's requirement that we don't redistribute 3rd party software, and satisfy our own needs to tweak our phones to perfection. Note that Qualcomm/HTC is interchangable with any SoC developer and any OEM. Rather than Qualcomm/HTC it could be Texas Instruments/Samsung, or xxxx/yyyy
Of course there are some other snags with this (WP7 Platform Builder is not publicly available), but one can dream right?
Side note, for more info on what a BSP is from the horses mouth, look here.