Having when editing 50% of the screen place occupied by the wiki source and 50% occupied by the preview could be cool. Especially if the preview is in a scrollable box, scrolled by default to the position of the cursor in the textarea and updated in realtime (This have to be optional since for slow computers it could be dangerous :D)
As-you-type previews with the current code is not something I would recommend, especially for long pages, as the script would run through the entire wikicode every time you press a key. Anyway, what's the big deal of hitting the preview button every once in a while?
Even if the preview isn't perfect the fact that it could be "real time" and on the client side make it usefull i think.
That's the whole point of it :o) I'll try to do it as feature-complete as possible, though, so it should be close-to-perfect in the end.
-Pedro