I special ordered my IS 350 C back in October and my sales guy said that 2014 is the last year for the convertible IS. He said Lexus would be going in another direction for 2015 and forward. I guess it will either be an RC convertible or - like Toyota did back in 2008 - no convertible available anywhere in the line.
Normally I would take anything a car salesman says with a grain of salt. When I was looking for a 2004 Solara convertible 10 years ago, the sales guy told me Toyota didn't make a convertible for the `04 model year. I just sold my `04 Solara convertible a month ago to make room for the new IS. Everything my guy at Kuni Lexus has said has been spot-on everytime so I'm inclined to believe him.