I've had the same questions. I just ordered my '07 IS250 with Luxury pkg and 18" alloy wheels, but I have absolutely no idea what my wheels will look like. Are they the same as the 10-spoke 17" wheels? The Lexus documentation is horrible concerning wheels.
On a side note, I saw some COOL wheels on an SC430 on the showroom floor. Bi-metal inset on 5 spokes. I wonder if it will fit the IS250. Mine is Smoky Granite Mica, so it would match really well.