But now there's a standard. jQuery will (most likely) support this, as will any other javascript library. Just include one file (that you're probably including anyway) and you're good to go. The cool thing is that now people with HTML5 browsers will have access to plugins that can extend the behaviour of these fields.

One plugin I'm missing, that this makes possible, is adding a "recent dates" box to the date-selector: when planning a vacation, I need to enter the same two dates (often 4-6 months in the future) on the websites of airlines, hotels, car rentals etc. over and over. That gets old really fast.