The project already exists: it is called http://wiki.maemo.org/Mer. Unfortunately, Maemo is currently transitioning from a Gtk+ based GUI to a Qt based one, so you'll find that you'll be nearly alone in trying to run the Gtk+ applications.
(IMO this is also what is hindering the Maemo/Meego platform: damn API breaks).

You'll quickly find out that there's virtually no useful application there that is not
a) A opensource application (so, port it to WebOS)
b) A proof of concept for something that is soon going to be released onto another platform
Forget about Skype, it's not made by Skype but by Nokia and is more a plugin than an application.

Opensource freeware for N900 is available in http://repository.maemo.org/extras/pool/fremantle/free/ .
Closedsource freeware for N900 is available in
http://repository.maemo.org/extras/p...ntle/non-free/ .
Closedsource payware applications..... well.

A few things.
Preenv only supports a small subset of Pre applications, namely, those that use the PDK, but are not plugins. Supporting a similar subset from all N900 applications would be trivial, basically because unlike the Pre you'll find out such a subset doesn't use any closed source library at all. In fact, the project would not be to run n900 applications on a Pre, but it would be about running plain GNU/Linux SDL applications on a Pre. As said, trivial! Even Pre engineers would help you out.

As the author of libSDL_gles which many free N900 games use, I can tell you that it very probably runs "as-is" on the Pre. If it doesn't, it can be very wall patched to run over Palm's SDL.

Now, for the difficult part: you want to do more than what Preenv is capable to do. Unlike the Pre, the N900 has a myriad of different toolkits and libraries an application can be using.
- If you want to run applications based on Gtk+Hildon, you would need to check on Mer as said above. Not everything about this toolkit is open, for example, liblocation (pretty much equivalent equivalent to WebOS's com.palm.location service); but Mer's mission was ~ to be "Maemo without the closed components" (Stskeeps is going to kill me).
- If you want to run applications based on Qt, just BUILD Qt for the Pre. THAT would help Meego-N900 crossplatformness.
- [Random other toolkits]

Btw, missing "X11" is not a minor problem at all. I hereby warn you =). Fortunately, you could do "something" by using Xvnc and a libSDL based VNC client. It would be slow, though.

Also, I saw you on IRC asking for the liblocation binary. Note that preenv uses _no_ library from the Pre's ROM. Running the N900's liblocation binary would be a big task while on the other side reimplementing it over com.palm.location services would be trivial. Do the latter.


As an ending thought, what reminded me about the similarities between the N900 and the Pre is that someone ported DrNokSnes to the Pre (another project of mine, btw).