Do you know how your site looks and functions in IE7? No? Better get testing: IEBlog : IE7 to be distributed via Automatic Updates!
To help our customers become more secure and up-to-date, we will distribute IE7 as a high-priority update via Automatic Updates (AU) shortly after the final version is released for Windows XP, planned for the fourth quarter of this year.
Emphasis added. Based on this, I would expect IE7 to be adopted pretty quickly even though users will have the option not to install it when it downloads via Automatic Updates.