You know, given the relatively simple complexity of this game (I say that more from a design standpoint, assuming everything is modular)… Have you ever considered porting it over to the Android platform for the higher end Android devices? Platforms like the Droid-X, EVO 4G, Galaxy-S line, Incredible and others have 1GHz processors with at least 256-512MB of ram. It seems feasible to me that this fun little game could be [relatively] easily ported (perhaps with some minor impact to visual quality) to a mobile touch based platform to wild success.
Especially when you consider how you’ve got a wealth of ideas here in your forums, you could request ideas for tighter, shorter (mobile friendly) missions that people could accomplish on their transits, and more extensive ones for use when at home or just bored around the office. I know the platform is lacking for something like this, and with the larger displays of those devices (almost all of them being 4" or larger) the sprites only need to get marginally smaller but will still look great. Applying your knowledge over to a different platform, particularly one with such a solid content delivery system and user base may help you to see exceptional growths in both recognition and revenue.
Anyhow, just a thought. But with mobile hardware clambering up exponentially, perhaps a new realm you can dive into… Great game by the way.