Also, the only way I can see Dylan fixing this without getting access to some Steam code that's likely proprietary, is to have a section of the servers tie your login name to the number of demo plays left. Everything using files that are on the computer itself won't work for the same reason the current way doesn't work. This fix would still be able to be worked around, though, you'd just need to create a new account with Audiosurf every time you wanted to play more.
Basically, the work required to fix it(or, even, to abuse it) isn't really worth it for the other downsides.
