I never had any problem running desktop Linux apps on my N900 using X display back. However, it never worked the other way around. Just like you said, the X11 forwarding doesn't do the trick when trying to run the Maemo applications on a desktop client.

The problem lies in the fact that most Maemo apps are written expecting a very specific OpenGL ES compatible display driver. I think it's a matter of the code being overly optimized.

Sometimes I can catch an error on the display like this:
Xlib: extension "Generic Event Extension" missing on display ":2.0".

Basically, the full X11 display on the Linux desktop is missing a component that the Maemo apps need to run on that framebuffer.