What is the best way to generate a random 2D heightmap for a tilemap?

I've been hunting for tutorials and getting kind of frustrated. It seems like most of the terrain tutorials are built around the already-existing terrain system. I have a much less intensive tilemap system that can accept a 2D array of tiles as it's input, letting me create multiple types of 2D terrain. I'd like to generate a island, and the best way I've found are heightmaps:

http://goandgetflucked.blogspot.com/2010/03/as3-dynamic-terrain.html

While this tutorial is in AS3, and can be converted for Javascript, it seems like it should be applicable to Unity3D as well, but I can't find similar results and functions don't seem to do quite what could be converted. I'm using C# - can anyone suggest a method similar to the above tutorial, or another better method?