There are under tile radiant electric mats, but I've never heard of one that only goes in the grout-- in fact, it seems to be that such a system would make it very easy for the grout to fail...

There is a wire type that works exactly like the mats except that it is loose wire layed down in a continuous loop. It doesn't go in the grout, it gets mudded in under the tile. The two systems work identically and both can be thermostatically controlled. Mine is programmed to come on for the morning hours, shut off for the day, come on again for bedtime, and go off again at night...thus saving on energy costs.

The advantage to the wire type over the mats is that you are not restricted to heating rectangular areas...you can cover the nooks and crannies, so to speak. Of course, most prefer to work with the mats for the ease of installation and convenience. Either works equally well, however.