I have a 1999 IS200 which 6 months ago developed the same problem. Took it to Lexus in Guildford and they couldn't suggest anything as the car was running perfectly apart from giving them loads of money to look into it.
6 months later and it's still running perfectly, but now I have a problem! MOT due in 30 days and new regulations out on Jan 1st 2012 means it will fail with those warning lights on! Can I disconnect them temporarily for the MOT?