I do like the ruggedness of bi-metal probe thermometers, but they are easily uncalibrated with use or abuse. Having a traceable calibration thermometer to check those bi-metal thermometers is imperative. On top of that, the bi-metal thermometers may not be as linear in their response as some other types, so having that calibration standard and calibrating at mashing temps is good practice.
With that said, those cheap electronic units that Sean Terrill mentioned months ago and Amanda pointed to above, seem to be a great deal and several are waterproof. I see that you can pick up a calibratable unit for $19 (Model: RT301WA) and its spec says its within 0.9F. That is pretty good since a much more expensive Thermapen is only slightly better (0.7F). Considering that the cheap unit is made by the Thermapen people, I would skip to the cheaper option without a blink!