Always the biggest false hood of any platform moving between architectures and one that continues to perpetuate throughout the Industry.
This depends on how well they do the API. The CLR versions of RT programs should have no issues. Flipping between architectures in CLR land works seamlessly. C++ will be slightly trickier with endyness but any decent C++ programmer will have no issue there.
If anyone can pull this off Microsoft can. If I went back 17 years ago and pulled up a program I wrote in Win32s (preview win32 lib that ran on win 16 3.1 prior to 32 bit window launch) and compiled it on windows 7 it would most likely work.
On the other hand if they do a crappy job with API parity or windows 8 fails on the desktop or the tablet then you are absolutely right.