Just to add my 10p worth on this one - my IS220d has this exact problem too. It is going tomorrow so I guess I don't really mind any more! But it has been really annoying because I do mostly listen to the radio. I just wanted to let you know that I had my car serviced a little while ago at Lexus Ipswich, and told them about it. They were extremely helpful, and the service manage came out to have a look. He told me (as others have mentioned) that the antenna is built in to the rear glass. Hence it happens when you turn on the mirros/rear windscreen de-mister. Note that if you have an SE-L (or other model with the heated windscreen - the Ford-style little wires integrated in the front windscreen, but only covering the bottom few inches to de-ice the wiper blades) then there is a separate button for activating this front windscreen heating, and therefore I really don't think that the windscreen replacement is connected to your radio reception problems. But then as others have commented, if the radio was definitely fine before, and instantly not right immediately after the replacement then maybe there is a link. But my guess is that it must be a coincidence.
ANYWAY - I digress! I just wanted to say that the advice from the service manager at my local dealers was to really (REALLY) polish the rear windscreen inside and out and get it as shiny as you can. He said this was a common problem on the IS, and that it was caused by muck on the heated elements. To be honest, I never really gave mine a thorough clean to the extent that he suggested, so I never did get it resolved! But I hope this helps you. Good luck!