An SR22 pilot encountered unforecast icing conditions; declared an emergency; and diverted to an enroute airport.

Narrative

Received a standard weather briefing from FSS via phone indicating no adverse weather along the route (except high altitude turbulence). The worst indication was 'widely scattered light rain north of ZZZ VOR.' Started to pick up ice north of ZZZ VOR while climbing from 6;000-7;000 FT. Requested descent back to 6;000 FT to shed ice. Request was approved but was advised by ATC that MEA south of ZZZ VOR was 7;000 FT. Decided to continue since conditions had been forecast to improve further south. Climbed back to 7;000 FT just north of ZZZ VOR. Entered moderate to heavy precipitation and ice began to accumulate again. Attempted climb to 9;000 FT but did not clear the clouds. No tops report was available. Decided to divert east so that an emergency descent could be done safely if needed. Descended back to 7;000 FT; still picking up ice. Accumulated about 1/4 inch of mixed ice and lost about 10 KTS of airspeed. At that point; the engine sensor unit failed; which resulted in loss of OAT readout and fuel totalizer. Flight could probably have been continued safely even under those conditions; but I decided to declare an emergency so that we could descend below the MEA to shed the ice. About 1 minute later; we broke out into VMC and descended visually to 3;000 FT; by which time all the ice had been shed. Diverted to ZZZ to refuel and see if the engine sensor fault would correct itself. (It did.) Landed ZZZ. Filed a pilot report. Resumed flight and arrived at destination without further incident.