After seeing it on a YouTube video, I've ordered an SF2000.  Physically it looks like an SNES controller with a built-in screen.  It includes emulators and full rom sets (!) for NES, SNES, GB, GBC, GBA, and Genesis.  Unfortunately the SoC doesn't seem to have the power for SNES and GBA, but at less than C$30 (shipped) I'm not going to complain too much.  And who knows, maybe the hacking scene will figure out how to improve performance.

But it got me to thinking about the "perfect portable retro system".  

• able to emulate all 2D consoles & handhelds
• six action buttons for Genesis (along with D-Pad, analog stick & shoulder buttons)
• start, select & menu buttons
• headphone jack and/or bluetooth headset (skip the A/V out)
• hi-res 4:3 LCD 640x480 minimum, ideally 1024x768 or higher
• SoC with built in scaling & window+border
• Decent menu with config options (e.g. button mapping, pixel/screen scaling)

Display is tricky for any multi-emulator because of the different resolutions and display and pixel aspect ratios of the original system.  Having a higher resolution gives more options.