Yeap, there is still the older Embedded Visual Tools that works. I still use them to write apps for HPCs and PPCs. For the most part, you just need the newer SDK installed for the Visual Tools to compile for the newer platform.
My own app is more or less cross platform from HPCs to PPCs all the way to WM5 devices. The only difference for my app between HPCs and PPCs version was the resource file and a bit of GUI code. Other than that, its just a recompile for the platform and cpu.
EDIT: Oh and about PalmOS (which is what this thread is about right?) ... its kinda good news for me. My Palm app which was written 3 yrs ago still works on the latest greatest Palm device and the same binary works from PalmOS 3.1 to PalmOS 5.1 (or is there 5.2?). Hopefully, when they release newer OS, they continue to provide backward compatibility.