Tom -
I have done a couple of concrete countertops for some rental properties I own. It is pretty easy to do once you grasp the general concept. I highly recommend the dvd instruction or book by Cheng.
If these will be your first attempt, I would also recommend you buy a premixed stain additive that you simply add to your cement mix. You will overpay for the premixed additive, but until you get some experience under your belt - it is the easiest way to ensure a uniform color throughout the concrete mix and will look more natural than a concrete stain applied to a finished concrete.
As with anything, the prep work is really the key. It can be done, but I also do not recommend pouring the concrete on to the existing cabinet bases (ie - poured in place). I would go the route of making the template/forms, pouring and then installing onto the cabinets. The template is the critical step. I used thin pieces of wood strip and a glue gun to ensure my templates where the right size and matched up to the wall imperfections/etc. This also allows you to build your forms to the exact tolerences of your cabinet and wall setup based off your template.
During the pour it is critical to vibrate the forms. You will still have small air pockets (pinhole sized) in the top of the concrete. You correct this by applying a slurry to the top of the counters and then wet grinding/sanding until you get the polish level you are aiming for. I would also recommend buying the premixed slurry on your first couple of attempts. The biggest reason is that you can buy it in the exact same color as your stain, so you know it will be uniform.
The other critical advice I would offer is that you just need to remember that any imperfections in your form (ie, undermount sink edge/cutout; dust, form seam caulking job; etc) will be transfered to your finished coutertop. Take your time on the form build and the rest is simply vibrate and then slurry and polish once the concrete cures.
Give it a go - nothing too hard about it. The best case is that if you mess up, you are really only out some time and money for concrete mix. You will also need a couple of cases of beer for your friends that are needed to help carry the completed counters into the house. The jokers are heavy.