it doesn't work with internationalized dates (no I18n widgets!)
it requires to manually replace every date widget (what about admin generator?)
Well, it's really an ugly solution.
So, I decided to write my own. I started from this demo and
near each of your date selects (just on the right of the year). The calendar png image (courtesy of famfamfam) must be in /images/ directory (unless you want to edit datepicker.js).
If you want a date select to be excluded (i.e., the calendar not diplayed beside it), just add a nocal class to year's select (see demo above).
If you need localization, just include date_XX.js (e.g. date_it.js for Italian) after date.js. You can get it for many languages at jQuery date methods.