I have the same problem with my 4/4 after a cold snap. It does clear itself but I am sure a hair dryer would speed up the process. It is definitely moisture vapour that causes the problem, as for a few days while it is drying out there is often water on the inside of the speedo glass which disappears over time - until the next frosty spell when it happens all over again.
In severe cold weather, sometimes the speedo itself also stops working, but always comes back on its own (so far).
I don't think a failed odometer is an MOT fail in the UK, not sure though. Mot is based on safety aspects and emissions only as far as I know.
