I just bought the Burley Honey Bee which does what you're asking. The front wheel stays on when attached to the bike and can convert to a stroller within a minute with no tools needed. You just loosen a knob and put the wheel up for when you're pulling it by bike or put the wheel down for when you want to use it as a stroller.
I ordered it from Amazon.
http://www.amazon.com/Burley-Design-...rley+honey+bee