Maybe a dumb question but does anyone here know if programs compiled on the Pi will run on the N900 (without cross-compiling) as
they are both ARM or are thet different architecture?

No, you can't run them here you are dependent on the OS.

You, quite clearly, have'nt a clue what you are talking about.
They both run the same operating system, Linux..
I'll wait for sensible answers...

It really depends on what version of Linux you use. Other factor for cross compilation could be kernel, drivers, ect... I have been very interested in trying to port Kali Linux to the n900 because they have been very adimate on making sure there is support for the kali Linux ARM architecture. Unfortunately their support has been more tiny pc oriented than tablet/phone.

I could run phone software on tablet, phone, or laptop but only one can take advantage of the sim card and broadcast to a cell towere. You could use the same phone program on all devices because they all have ARM processor and matching OS. but you can not make working calls.

Did this help?

Look, this is quite a simple question. Perhaps I didn't make myself clear.
Suppose I wrote the classic simple, one liner "hello world" program (in 'c'),
compiled it on the Pi, would it run on the N900? They are both Debian based Linux systems. The only problem I see (at this low level) is cpu
archiecture. I can't test this at the moment. Does anyone here know?

A running program is a question of using the right OS (the executable file format - entry point, relocation tables and the like), the CPU (instruction set) and external libraries (if any). There are other considerations (memory layout, peripherals...) but those are nowadays abstracted by drivers and hence can be covered with the three thing already listed. To create a "hello world" app, you have the first two and do not need the third. So I would imagine it should work. But that is just my opinion. I have no way to test it either.

The N900 is ARMv7 where as the Raspberrypi is ARMv6,

Most rpi distributions use hard float which means binaries won't run on n900. If you use an armel distribution (or use multiarch), some binaries could run, but you would probably end up with unresolved symbols / wrong glibc hell on anything more serious.

I often compile stuff on Sheevaplug running Debian and often the binaries run just fine on n900. So you can run older arm binaries on newer but not vice versa.

I'm just get my raspberry and i put raspbian on it, i see it has support for
Python 2.6 and 3.2. So i guess it run the same code you just have tk compile for each device specificly. I did not have too much time to play with, i keep you update.