Changing resolution on the fly ....

Hi,
I am knocking my head on the wall for days trying to find out how to 
change the resolution of the Xipaq (and so the GPE that lays above) on 
the fly ...
When no application is running, one can change the resolution by using 
fbset, but as soon as an application runs, fbset has no effect until 
that application is killed. Can anyone explain me why?
Since Xrandr works fine, I think it is possible to change the resolution 
on the fly. The only problem is that only one "randr size" is set (the 
one given by the framebuffer when  the first Xclient is launched). I 
looked in the Xrandr library, but found no way to "register" new sizes. 
I've also looked in the sources of the randr extension of the XServer. 
Apparently, the function "RRRegisterSize" is used once when the default 
screen is used with the default size values, thus enabling only one 
size. I'd like to change and register different sizes, but have no 
experience in X development.

I am working on a driver that allows different resolution for the iPAQ, 
and I need a window  system that can also be resized on the fly in order 
to carry out further experiments ... Correct me if I m wrong, but Opie 
has no such capabilities, right?

Any help would be really appreciated

Armand Vedadi