I've read on other boards lots of speculation (special parade, drawbridge lowered each day, new fireworks, special projection on Castle to dress it up) but no specifics.. the big announcement about the 50th is planned for May 5th (4th??) this year (happily coinciding with Tower of Terror opening).
They have been doing LOTS of refurbishment to make the park look spiffy for the media event. They ripped out some trees in the hub - better sight lines to the castle, painted lots of Main street buildings, repaired /painted bathrooms, etc.. The Castle intself will be under wraps for most of the summer in prep for the 50th!