Well sort of. I think Sig is selling a 220, and a 226 frame with the 22 conversion already on it. It comes with a coupon that will allow you to purchase the centerfire upper at a later date, at a discount.
Kind of like buying the 226, then the 22 conversion, except backwards.
Anyway, I don't think it's been out very long, so there might not be a lot of people out there that have experience with it.
I have the 22 conversion for my 226, and I am very happy with it. I have about 500 rounds through it, and except for 4 or 5 problems, in the first 50 rounds, it has been perfect. The only real complaint I have heard is that the slide does not lock back after the last round is fired, so you have to count to 10. If you dry fire it, you can cause some peening of the chamber when the firing pin hits the edge of it.