This is not really a how-to post, more like a how-we-did-it post. I think I first heard of someone having a concrete counter top about four years ago, and they seem to be getting more and more popular. If I understand correctly the most common way of doing them is to pour concrete into a mold, and then install the countertop as a slab after it's hardened. This gives you a nice flat finish, plus you can get a nice decorative edge from whatever mold you use. You can also set up a concrete form, and pour them in place, removing the form after the concrete has set.
Or you could do what we did...and of the four or five countertops I've seen, none of them are done this way.
There is some wire mesh embedded in the concrete to strengthen and stabilze everything. Notice that the concrete is green. We mixed some pigment into it so that it didn't look as much like a sidewalk. The blue tape around the edge is masking off a wooden edge. Essentially our concrete form is the finished edge of our countertop, which is a piece of wood that matches the cabinet trim. 
After the "forms" are full of concrete everything is troweled smooth, which is actually a rather long process, smoothing and resmoothing as the concrete sets up. The hole in the middle of the countertop is were our sink is going to be, that wood is temporary and will be taken out before the sink is set into the counter.
The little blue squares are where some tiles have been set into the surface, the tape is supposed to protect them from getting covered in concrete.
And it more or less successfully did so. The tiles are from some artist friends, who's company, Earthen Craft Pottery, can be found here. Click on over and give them some love.Now that everything has been poured and has had a chance to harden, Eli and Trevor have scrubbed it clean and ground the surface a little to smooth it out a bit more. Then they are going to seal everything so that oil and such can't soak in and stain the surface. Voila! Hey-presto, a countertop.
