I know... there's one in CSS Form Builder already.
Well that's true but if you have a set of Insert / Update forms already built using data Assist plus the form fields are all associated with a database then I believe I'm right in saying that adding the date picker means losing all those associations. And having to put them all back by hand will be very tedious.
At the moment I'm using a jQuery UI and it works very well but how about building the date picker into DataAssist on the next version. Or even making a stand alone extension that wont interfere with all the data bindings that already exist.
A free standing or integrated (into DataAssist) date picker that allows you keep data bindings and easily format the CSS would in my opinion be a worthwhile addition.