Preview post before actually posting it

[SoonDead]

Like in the old forums. The preview button directs back to the post form with the same content, but under the form, a preview of the post could be shown.

Also it would be nice to have the preview available through ajax without the surrounding layout, as I've added an experimental markitup editor to the forum post forms, it works relatively well (still needs some design changes and fine tuning), and has built in support for submitting the form through ajax for previewing, and outputting the result in a window/iframe.