I basically followed the custom firmware instructions here: http://wiki.maemo.org/Modifying_the_root_image (this was a few months ago, so I don't remember exactly what I did).

The gist of it went like this:

1.Use the flasher utility to unpack the FIASCO image for OS2006
2.Mount the root image as an MTD device, and copy its contents to a temporary location (I used /home/maemo/root on my laptop, but it doesn't matter that much)
3.Make changes to the root file system as needed (see below)
4.Pack the directory structure back into an MTD image, and flash it onto the device.
(this was all done as root on my gentoo laptop btw, although it may have been smarter to use scratchbox - I don't know how to use it that well yet)

To enable usb networking, I followed the these instructions, but on my unpacked root image instead of the device itself: http://wiki.maemo.org/USB_networking...B_network_mode
I also added a symlink:
so that usbnet would start automatically.

I installed x11vnc by downloading the .deb from one of the repositories, and unpacked it into the root dir using "dpkg -x x11vnc.deb /home/maemo/root/". I don't recall if there were any dependencies required, but if so, you will have to install them manually as well. (dpkg can tell you what deps are required)

To get it to start at boot, I added a small script under etc/rc2.d/S99x11vnc (don't forget to chmod +x)
I also added fanoush's SDHC kernel and wifi patch, just for good measure. There was some trick to getting past the initial setup (since this is a fresh firmware, it will ask for things like time zone, hostname etc. on the first boot). I don't think I used vnc to do it as many things are still unset.