We are switching to using SDL 2.0 in the unstable builds. Thus far Doomsday has been using SDL 1.2, and it is presently only used for joystick input and the SDL_mixer audio plugin. When trying the next unstable builds, please let us know if you encounter any problems wrt. missing DLLs in the deployment or any other problems.
If you happen to be making builds from the master branch, you'll need to update your config_user.pri and define the SDL2_DIR (or SDL2_FRAMEWORK_DIR on Mac OS X) and SDL2_MIXER_DIR variables. (These are just like the previous SDL config variables except prefixed SDL2.)
In the future we might look into utilizing SDL2 also for keyboard input on Windows, as there are a number of known issues with Qt key input when it comes to game-style interactions on that platform.