I reckon Half Life 1 could be done, albeit with a drop in resolution & adjustments with the engine of course. It won't ever happen though, publishers don't seem that keen on doing hard core games on the DS unless its of Japanese origin. The problem is that people don't buy them, take away flash carts and we'll probably have more.
The only problem I've had with DS FPS games have been with level design (no fault of the DS) and of course hand cramp.
I don't think Moon will be "the DS Half Life", I expect it to be a solid game though. We may see something along HL quality, with story, level design and gameplay, its certainly possible.