My experience also with the Cat-Eye wired computers. I have them on every bike, including two beaters that are only ridden in the rain, snow or other lousy conditions, and none have ever failed in any way. I've always thought you could mount these things on a submarine and have them work reliably. I see no reason why Cat-Eye wouldn't seal the wheel sensors just as well.

One thin I do is dab some silicone grease on the handlebar mount contacts. It keeps out water and grit and has no effect on the readings.