I've got a Specialized wireless computer. The fork-mounted sender is totally encased in plastic, aside from the battery door located at the bottom of the unit (facing the ground).

It's zip-tied to the bike. Can I get this wet while washing the bike, or should I cover it with Seran Wrap or a bag? I don't want to cut the zip ties and re-mount the darn thing every time I clean the bike.

I don't know for sure, but generally the better computers will be sealed against rain. As long as you don't put a heavy spray on it, you should be OK. If you want to be safe, wrap it up with plastic and a rubber band.

I spray mine with water plenty and it's been fine.

OTOH, the magnet assembly has rusted, and the display once was fogged inside and stopped working.

Thanks guys. I just got back from a 15-mile ride and washed the bike. I removed the computer from its mount, but left the sender and magnet on the bike. Everything seems fine.

It's an outdoor product. I wouldn't worry about rain or wheel splash or normal washing but I wouldn't submerge it in a lake.