I'm considering buying the n900, but I have some questions about running regular linux softwares on it:
1. If I have a debian installation file on my pc of lets say open-office, and I want to run it on the n900. so, can I simply copy the open-office installation file to the n900, install it, run it and that's it? Was it possible with the n8x0?
2. What "cross-compiling" is and how it differentiate from "porting"? Do I need to cross-compile every application?
I don't know enough about it so I thought you would be able to help me. I wasn't sure where to post this thread, I hope it's the suitable place.

No, this will not work. It wasn't possible on the N8x0 either.

The CPU in the N900 is ARM CPU, while the CPU in your PC is most likely a x86-compatible one. Cross-compiling means compiling a program for another CPU architecture, e.g. compiling programs to be run on ARM on your x86 PC.
Porting is modifying the program code to work in a different environment, e.g. to adapt a user interface from mouse usage (3 buttons) to touchscreen usage.

...but it's possible to install and run debian on the N8x0 and debian has installation files for the ARM architecture... Given that, would the answer to 1 still be negative with regards to the n8x0:s?

Well you can't just take arm deb's off of debian.org and expect it to work on Maemo. Doesn't work for Ubuntu either for more complicated apps. Both Ubuntu and Maemo are Debian derivative, but they have a different set of packages so dependencies are different. In the case of Maemo, maybe even a different gcc version.

But you can take your average linux app and compile it on Maemo. Its about the only phone where you can say that.

a) you are nitpicking and
b) yes, to this very specific question the answer would still be "No" unless his desktop has an arm processor

Thanx!
Is it possible to cross-compile apps on the phone itself or only on a computer? hardware wise and software wise? (cross-compiling the cross-compiler)

Does the cross-compiler open source?

The cross compiler would most likely be gcc, so yes, it is open source. It's just a matter of telling gcc to compile the source for the target architecture (in this case arm) instead of x86.

And also, how tech savy do you have to be to cross compile programs/packages? Is there a standard set of instructions non-techie windows users can follow?

Well, perhaps it would also be appropriate to mention, though, that compiling programs for your N900 is not exactly a walk in the park or anything that a normal user is "expected to do". This is not the way users should install applications and, frankly, it is well beyond the capabilities of most users anyway.

If you're interested in getting some of your favourtie Linux software to work on the N900, the best thing to do is to first of all wait until the device comes out (any day now..., and then start following these forums and the Maemo.org downloads lists to see what is and what should be up on the repositories soon. The repositories are how software should be installed. If you're thinking of a program and wondering why it's not available, start a new thread here. Somebody else, more tech-savvy perhaps, might get interested and start working on it.

So basically I suggest you browse what is available now and see what might be available soon. Don't assume that just because a program is available on Ubuntu/Debian, it'll be easy to get to work on the N900.