Well the Pandora uses the same CPU as many cell phone devices out there, if he can get it running on their browsers or something by compiling it maybe he can get it running on ours so it's not just 4000 people playing it its all the smart phones.
True, more devices might be more enticing to a developer, but just because a device uses the same processor it doesn't mean it'll run the same. If he decided to build a version of the game for ARM-based devices running something like Windows Phone 7, he'd have to make sure it runs on every WP7 phone he says it'll run on, and let the rest take their chances. There are at least 9 phones out there right now running WP7, and I believe it requires a 1GHz minimum processor speed, certain graphics processor, a minimum memory size, and so on. If Notch says "now Minecraft runs on WP7," he'll have to make sure it runs on those phones, 'else face the wrath of many disappointed (possibly paying) players. That's a lot of work for one guy.
Of course, I'm talking theoretically. He could restrict it to individual phones with hardware that he knows could run the game, which would be ok, but it would likely cause a lot of bitching to fly his way one way or another. Also, from his blog it sounded like he's been in the process of or already has opened a studio somewhere. It's entirely possible he could hire people to handle ports to other platforms.
Another thing I noticed is that the game runs like utter dog poo on 2 out of 3 of my computers. The oldest (5 years old) runs it the best. It has the strongest graphics card in the lot but a weaker processor than the newest, which has a Core 2 Duo 2.53GHz and a 9300GS graphics card. Of course the strongest graphics card should run it the smoothest, but a lot of stuttering and lagging seems to be caused by the reading and writing of the chunks of level data that the game is constantly producing.
I wonder how the game would run if the data were being read from and written to an SD card instead of the hard drive? That might be a good test of what I feel is one of the biggest performance problems the game has. I could try that later when I'm not as busy.