My IS220d has been blowing cold air for the last two years but because i only use the car on weekends I lived with it until this winter when i nearly froze to death!
Read other posts refering to head gasket failure but thought that wasnt likely because the car was driving fine.
Anyway took the car in to Lexus wolverhampton who confirmed within an hour that the head gasket had failed. All they asked me for was the service history and as the car had only done 78000 miles it was under warranty.
Lexus have fitted a new engine block and given me a free valet. I had the alloys exchanged and now my car is as good as new.
Fantastic service, very happy.
Advice: If your heaters are blowing only cold air get it checked out before your do over 111800 miles.