5.9 adds three additional themes - Orange Crush, Fire Engine Red, and Purple Sage (phone only)
+ Honeycomb tablet improvements for viewing photos, TweetMarker, Instapaper/Read It Later
Schedule tweets to send later
Several new Themes and Settings
TweetMarker service keeps your place on multiple devices
Zip It enhancements and settings
Photo enhancements, including via.me support, pinch zoom, save pictures to device
Signature option lets you sign tweets
SSL support