I'm personally waiting for the Pandora, but I'll throw in my 2 cents worth of opinions on the DS and GP2X as well...
The GP2X has near perfect emulation of NeoGeo, NES and a few others, good support for older arcade games through MAME, a fairly competent SNES emulator and the best Genesis emulation on any platform IMHO (of course, that's ignoring the fact that 32X emulation isn't present, but SegaCD is). GB emulation is a joke, but GBA emulation has become fairly usable with the latest patch to gpsp2x. If you have a F-200, ScummVM is a really good package as well. Using standard SD cards is really nice, too. But the GP2X is lacking in terms of good controls (esp. the pad/stick seems to be a source of frustration for many GP2X owners, though I myself find it accurate enough (I have a F-100 MK2)).
The DS has an edge over the GP2X in GBA emulation since the DS can run GBA games natively at full speed - but this requires a Slot-2 backup solution. The DS has an excellent library of commercial games. Emulation and homebrew requires a flash cart (and a compatible Slot-2 expansion if you want to run GBA games), which uses MicroSD cards which can be fiddly at times. NES emulation is very good just like on the GP2X but suffers from downscaling of course. The downscaling is the biggest gripe I have about the DS, since everything but GBA and native DS games must be downscaled to fit the DS small screen(s). I find SNES and Genesis emulation lacking on the DS, mostly due to downscaling.
Bottom line: If you want a good library of commercial games, and perfect GBA compatibility, and don't mind shelling out on a good backup solution, the DS is for you. I'll probably be buying one for just these reasons (I'm still getting a Pandora, mind you). If you want NeoGeo emulation as good as it gets on a handheld, the GP2X is currently the best of the crop, but with the Pandora just around the corner promising fullspeed SNES and PSX emulation with dual analogs and a physical keyboard for easy computer emulation or on-the-go Linux computing, I'd personally wait for that one.