Just FYI, flasher on Windows can't enable R&D Mode last I checked. You'd need to use a Linux distro's Live CD an run the Linux flasher from that.

ALSO, abill_uk: Have you ever talked to, or heard/read anyone mention, how their N900 acted when blacklisted? There is no way in hell I can see that logically fitting blacklisting - because the N900 can boot without a sim card and without a functional cellphone plan. (Just in case you are right - guy with the problem, can you try booting without the sim card inserted and see if it makes a difference?) He also said that they checked with Vodaphone already, and they don't have the IMEI blacklisted.

Do vi_'s suggestion please - post the output of the flasher. ALSO, try reflashing the bootloader. (I don't really know much on the subject of using flasher for that, though.)

Other than that, hardware damage is possible. You can also try to use framebuffer kernel, flash it onto the device using the --kernel flag on the kernel image, and see if you can tell what the problems are - if you get all the way to loading dots, framebuffer console SHOULD show you the errors (though it might take a bunch of tries at seeing the errors and reading them fully in the rapidly scrolling console output).