This isn't quite right. It requires a complete rebuild of the OS. On your desktop there is x86 and AMD64.. so the OS will still run.

OMAP-2 to OMAP-3 requires a complete recompile.

Then you have The 3D. The N810 only supports OpenGL1.1 (IIRC), but the foundation of Maemo 5 is OpenGLES 2 (IIRC, version numbers suspect.) - meaning that the hardware of the N810 simply not only requires a complete recompile of Maemo 5.. but an actual complete re-write because of the 3D requirements.

Think of trying to run Crysis Warhead on your old Pentium 400mhz.

Now.. with all that said:

I was and am hugely against the decision to not bring Maemo 6 to the N900 because from everything released so far (M6 device being based on OMAP-3 as well) - there should be no reason it can't run on the N900.

However, Qgil asked me and others in another thread to hold off until the release of more M6 details before passing judgement and I plan to do just that.

From what little we know so far... at the very least it appears that the QT libraries are going to be supported for M5 for sometime and M6 is entirely based on the QT libraries. This means that even though the OS itself may not be upgradeable... the software written for one should work for the other.. barring certain hardware incompatibilities such as Multi-Touch (which M6 has been said to support, since it will be based on capacitive touchscreen).

All information above taken from about 100 (approximate, give or take 95 or so) different threads already written about this topic from these forums... and may be perfectly accurate, somewhat accurate, or wildly inaccurate.

Take it as you will.