The enclosed patch adds a nice jquery-based calendar selector control to the Post Timestamp box. It's based on Ted Serbinski's jCalendar, with minor modifications to avoid clashes with prototype:
http://tedserbinski.com/jcalendar/index.html
Some further work is required (notably localization), but I thought I'd post it here for feedback before I spend more time on it.
Notes:
I chose jcalendar rather than Kelvin Luck's datePicker because datePicker only appears when you click on an icon. jcalendar gives a visual calendar view of the date at a glance.
The new input_dropdown() function in wp-admin/includes/template.php might be useful for other things. Perhaps it belongs elsewhere.