We haven't tried running F1's in the wet but we have run touring cars and off road trucks in the wet on asphalt. Here is what we've learned and also what we've found on the web about running F1's in the wet.

Photos

A very cool photo gallery of running F1's in the wet on the UF1 site.

Wet running in the F1 Italian Series.

Waterproofing

• The best option I found was to buy waterproofing tape from a hardware store and wrap it around the ESC, receiver and other electronics. It's much tidier than a balloon
• A number of manufacturers make waterproof ESC's and servos including Traxxas, LRP and others
• An old silver can motor is a perfect wet weather runner. If using a brushless motor use a sealed one rather than one with lots of air vents (which will obviously let the water in)

Remember when sealing electronics that this also keeps in the heat. Not a problem if it's cold and wet but if it's hot and wet it's something to keep in mind.

Videos

Here's a very cool video running on a track that's very wet and had snow cleared from it:

Which Tyres

Use the softest slick tyres you can find. For example:

• Tamiya F104 - Pit Shimizu Softs (571 front and 575 rear) work well. Ride and Pit Shimizu also make soft grooved tyres but I haven't found any wet weather recommendations on forums for these or tried them.
• Tamiya F103 or 3Racing F109 - Pit Shimizu softs work well. The consensus on the forums is to use the TRG Superion tyre (rear #TRG2400 and front #TRG2300) or the Cross Soft (CFT-4UM-S and CFT-3UM-S) - which appear to be the same tyre as the Superion.
• 3Racing FGX or HPI F10 - Pit Shimizu soft grooved work well 

Some people make their own wet weather tyres by starting with soft slicks and cutting grooves into them. You can see an example of this here.

After Run Maintenance

Here's the not so fun part. Water gets everywhere! In particular it will probably get into your bearings. You can try and keep it out by spraying with WD40 but this is not a perfect solution. You can also use rubber sealed bearings rather than metal sealed bearings. But water will probably get in anyway if the track is really wet like in the UF1 photos above.

The best advice is to strip your car after running in the wet and dry it thoroughly with a hairdryer. As the off road guys will tell you letting your car dry naturally is the easy solution but not the best for the life of your parts. Of course I'm too lazy for that and just replace the bearings at some future time.

Water will probably also get into your tyres and soak into the inserts. Once this happens the tyres become very unbalanced and are really only useful for wet weather running. To minimise this make sure the tyre is properly glued and cover the air hole in the rim with tape. After cutting open tyres used in the wet we were blown away to find a lot of little stones in them from the track (covering the air hole with tape prevents this).