Stu had suggested my checking the Davis site for driver upgrades as I've been having occasional problems with the raw rain data failing to zero, and feels it may be a comms fault.
I checked the Davis site, and they list the latest Silicon labs driver as being Version 2.8, released Dec 2011, which is the release I downloaded when I started up mid Feb.
I've checked the Silicon Labs site, they have a new release, version 3, released on the 27th March which "corrects issue where transmission would stop when a 0 length packet was sent incorrectly on aligned packet boundaries".
The link for the updated driver for those interested is:
http://www.silabs.com/products/mcu/pages/usbtouartbridgevcpdrivers.aspxJC