[raldi]

The solution already exists:

http://en.wikipedia.org/wiki/Thermostatic_mixing_valve

[savvyraccoon]

I have one from Delta - http://www.amazon.com/T17T051-SS-Dryden-Tempassure-Series-St.... It works like a charm.

[stronglikedan]

Agreed, and as a mechanical solution, it is likely better than an electronic solution in a damp environment such as a shower.