I am not entirely sure what section to post this is, but given the assumption that people in the Open C forum are probably the ones most interested in portability of "core engine" or "legacy" codebases, this here seems to be the most appropriate audience for now.
This is kind of half question, half suggestion to Forum Nokia on a subject that will probably receive increasing attention over the coming year as the transition to WP7 is made.
One of the big changes (at least from what has been known so far) coming from WP7 is the list of supported languages. At least today, Microsoft declare only XNA and Silverlight as supported development frameworks, and C# as the only supported language. While this may sound a bit similar to C/C++, at a detail level this is actually a very different language, especially in its "managed only" form that comes with more Java-like features such as Garbage Collection and handles instead of pointers.
There are some other C/C++ variants that Microsoft experimented with on the CLR (the runtime interpreting the "bytecode" that C# and other languages get compiled to in a .NET world), but as the answers on this thread show
the restrictions to C/C++ code that can be ported in "/clr:safe" mode are actually quite severe, as it means avoiding many of the features of C that can be potentially unsafe, even just theoretically (i.e. you'll have to be able to convince the compiler that your code is not doing anything nasty, not some higher-level review):
Even then, the combination of using C++/CLI to target WP7 is not currently supported by Microsoft.
As a result, I believe that one of the most important tasks for Forum Nokia over the coming year is to give developers guidance how to ensure that their code remains portable between the C++ centric Symbian/Qt codebase they are developing today and the future "managed C" world in Windows Phone 7. With the (now aborted) switch of the Symbian UI from Avkon to Qt there was at least still the assumption that "core engine" code could remain common between "classic" Symbian, Symbian+Qt and MeeGo, but as of today the development model of WP7 will imply major source code break at every level of application development.
UI design will probably have to start from scratch in any case, but if even valuable core algorithms (game AIs, image processing, file format libraries etc.) have to be rewritten and re-tested from scratch, the effort to port "valuable" pieces of code will go up significantly unless portability beyond the Open C/C++ level is considered early on.
So the time to prepare for this is probably "now", and not when the first WP7 devices start shipping.