Now that I have my heightmap done, I have what I need to create a landmass and an ocean (or something equivalent, in case you're creating an extremely unique world). From past experience, creating the shape and size of the landmass and the oceans is actually a pretty difficult task. I tried doing it manually with varying results, and I also tried several photoshop variants, most of which were offered by the nice people at The cartographer's guild. And some techniques were brilliant. Few of those that produced good results were easy, however.
With my heightmap, every point in my map is a value between 0.0 and 1.0. By applying a filter, I can say that everything from 0.0 - 0.5 is ocean (black in the image shown), and everything from 0.5 - 1.0 is land (white).
I like the result. It produces no visible artifacts, or any bias towards any specific direction.
This could hopefully be useful even for those of you who like to handcraft your maps. With that magic wand, you should easily be able to split land and ocean into two different layers in Photoshop.