No. Even in the dysfunctional world of Windows, hardware platform is significant. The only somewhat blurred exception is between x86 & x64 platforms, where the latter OS will commonly run software compiled for the former (provided all dependent libs are present, of course).

You are partly correct in that the sourcecode is largely agnostic. Simply retarget your new hardware platform and hit compile....and monkeys might fly out of my butt

I'll offer you one concession - due to some of the limitations already mentioned, a full installation of, say, Gnome is out of the question. Therefore you cannot simply recompile Firefox as it won't have the application framework it depends on. That's why the Maemo5 browser is based on the core mozilla technology - the part that counts (I believe)...therefore pages should render identically to Firefox.

So, one real limitation on apps retargeted at the Maemo5 platform is the outer 'presentation layer' code. However, this in no way means that Maemo5 is a 'lesser' linux than Fedora etc - it simply has a different application framework better suited to the limitations of the device. QT and GTK and Tcl/Tk are all different frameworks, but we don't start criticizing the underlying OS for their incompatibilities, do we?