I am building a thin-client like an X-terminal which boots from PXE, runs an X server, opens gdm session and displays some statistics page in a browser.
So far it boots and runs X server and gets the screen attached to server by XDMCP and displays Gnome logon screen but I want it to log in automatically or attach to some session and I dont know how to do this

The idea is to have a bunch of such dumb devices, like a platform to run various info displays, barcode access stations, etc - plug it in, it boots small linux from PXE and after 30 seconds it comes up and displays a screen with specific app or a full screen browser page according to configuration for each MAC address of the device.

I dont want to run apps locally as the thin-clients themselves are too old and too weak for other than presenting a screen.