There are two forms of autocompletion that are already working (to my knowledge).

The simpler kind is akin to ‘snippets’ in TextMate: type a PlotDevice command's name (without parens) then hit tab and it will fill in the default arguments. You can also hit option-tab while midway through typing a word to get a more IntelliSense-style dropdown menu with completions matching the current word.

@samizdatco Wow! That a nice news. How we can add it to the manual?