I have just had this same problem. I have 2 Foresters .56 plate sg Turbo manual Ive had for 6 years and no problems (apart from a new radiator)
Ive recently bought another one, a 55 plate 2.5 turbo auto. It needed a few bits doing, which the garage got done by a Subaru specialist.
The car drives beautiful ,and has only done 88000 miles, compared with the other old girl whose done 124000, a lot of towing.That also drives nice.
This light came on last night when i started her up after washing her. Also the cruise light came on ,which I couldnt switch off .A bit weird.
I've read in the handbook it could be a problem with the emissions control.
Should i take it to my local garage, or is it something I can check myself?