As much as I would absolutely LOVE to see HL on the N900, I don't think it's ever going to happen..

The Half Life engine began life as the Quake engine, but it was heavily modified to the point where the standard open-source Q1 engine wouldn't be sufficient to run the game at all.

You'd be looking at basically recreating the game from scratch -- engine, models, textures, sounds, etc., and I don't think Valve would be terribly happy with the copyright violations that would entail.

Then again, it's Valve, so they might swoop in and hire the entire team making the port