autocmd FileType cmake set omnifunc=cmakecomplete#CompleteNow when you edit a CMakeLists.txt file, it should do omni completion when you summon help with the Ctrl-X Ctrl-O enchantment. Maybe someone will find it useful.
I've only tested this with the latest CMake and Vim versions, it is quite likely that I assume things that do not work in earlier editions of these programs. Patches welcome ;-)
Edit: Updated to add property and module completion