My wife and I also want to explore building our own concrete counters with built in sinks. There was a recent TV episode from Dirty Jobs, where Mike Rowe helped build one. Also, we have seen a segment of Rock Solid, where those two cousins built concrete countertops.
Watching these segments has really got me intrigued. I do know you begin with a melamine (sp) sub-structure, and wax the inside for ease of removal. The edges are caulked to give it a nice round edge, but is later worked on after the curing process. The concrete has fiberglass reinforcing, and both segments had wire mesh and rebar for strength. Once the colored concrete is poured, vibrators are used to remove the air pockets. After curing, the mould is removed and then there are several steps of polishing, with a final coat of wax.
This is about all I was able to gather from these segments. I know it is a hard process, and is built upsidedown, but the finished product looks great. I hope someone with some experience can chime in, in order to fill in the gaps I left out.