shimano m324s are the pedals you are talking about, with cleat-things on one side and regular pedals on the other. they are not the lightest pedals around but they increase the versatility of your bike considerably and I used them for 5 years while I was SPD'ing. Now I use Crank brothers system, and while I like it a lot more than SPD, CB doesn't make a pedal that can be used as either flat pedal or cleated. I have compensated for this by increasing my number of bicycles
.
That all said, it's also really easy to switch pedals once you get the hang of it. It's feasible to use clipless for longer rides and your current flat pedals for 'round the town...assuming your bicycling activities are that distinct (mine never seem to be).