Butch Curry over at Zombie Nirvana Games has another excellent and easy way to create random land masses in Photoshop. He details it in Podcast #14, but it goes like this:
• Create a new document
• Filter > Render Clouds
• Create new Fill-or-Adjustment Layer > Threshold
• Drag the little triangle at the bottom of the dialogue box back and forth until you've got something you like, and voilá!
In the podcast he goes into further detail about how much more editable this method is than merely rendering clouds and going to Image > Adjustments > Threshold. It's definitely worth checking out the whole tutorial! It can be downloaded from iTunes or you can watch it here on YouTube:
Fantasy Cartography with Adobe Photoshop #14