I did this to the kitchen counter in my last house. I used 12 inch marble tiles. That was three years ago (and no problems with the installation). I put down ¼ inch cement board bedded in thinset cement and screwed it to the counter, filled the seams where the cement board sheets met with thinset cement and fiberglass mesh tape, and then tiled right on top using thinset cement . The counter had a curved edge, so I used wood, but if your counter top is square, you could easily use tile for that, too.

There are alot of if's here.

IF there's no raised drip edge.
IF the edge is square, instead of being rounded over.
IF any corners in the countertop are square.
IF the formica's in good shape-- no cuts, chips missing, screw or nail holes, splits, etc..
IF the only thing wrong with it is that it's a nasty color.

Then yes, you can go DIRECTLY over it. Use an unmodified thinset mixed with a liquid latex additive.

Back in the 1960's, I worked in a cabinet shop and we did tons of work with high pressure laminates. For what Bill recommends, I suggest first scuffing the laminate with a power sander using 80 grit abrasive to get rid of the glossy surface.

Absolutely.