I keep meaning to write a blog post with some details on this, but as with many things, I usually end up coding instead.
Sublime Text 2 is almost entirely C++ (with a smattering of Objective C for Cocoa and Python for plugins). Coding is generally fairly straight forward: code on one platform (mostly Linux at the moment, but I switch around frequently), and then make sure it still compiles elsewhere.
Sublime Text 2 itself uses a custom UI toolkit. There are a lot of apps where this may not make sense, but it's not such an unreasonable choice for Sublime Text, where I always knew that a lot of the UI controls were going to have to be custom no matter the toolkit (e.g., the text control and tab controls). The UI toolkit sits on top of a cross platform abstraction layer, which is more a union of platform functionality rather than lowest common denominator.