Matt: the N900 already has video output; connecting by BT to an arduino to get similarly poor (NTSC) video seems overly complex; can the N900 video out be manipulated directly by N900 firmware, or is it tightly coupled to some sort of video/picture playback hardware?
(I think it'd be a neat hack to throw up some sort of hackable interface (BASIC, FORTH, BitLash, C64 emulator, whatever) on a big NTSC screen from the N900, but OTOH that pretty much has significantly lower resolution than the LCD, so perhaps not.)