I've been wanting to do this since day one, the wife likes the look of the factory 255/50/19's and wheels.
As we bought our '11 new in July of '10 and have just over 12k miles on it now, I'm thinking come spring might be the perfect time for a nice set of summer 20's and keep the 19's for the winter as they'll be needing new rubber around that time.
I should start back up the conversation with the wife again and get her interested in the 'nice looking BMW 20's' that are available.
This is a good post with changing out the wheel arches.