My wife needed a source of water for a speciality diet food - let's just call it "Optipuke" - that was at a temperature of 50 degrees C. Any more, the medical qualities of said food got lost. Any less and it tasted somewhat tepid and even worse than usual. Kettles seldom come with a temperature readout, so I purchased a US$8 probe thermometer rated at 150C from DealExtreme (I have an addiction). Fortunately our kettle fills through the spout, so the lid was free to play with. The only other thing I needed was a 10mm long piece of silicone tubing (rated to 300C) that just fitted over the probe.
I took the knob out of the lid, which left a hole that was conveniently just the right size for the thermometer. I then inserted the probe and slid the piece of silicone tubing firmly up to the top . This is essential to stop steam coming out of the lid hole, and stops the display rotating when you move the kettle. The probe I bought seems to be reasonably resistant to steam, and so my wife's dietary needs are now met.
As a bonus, when the diet is done I can dismantle it all and still have a functioning kitchen thermometer.