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.
(Via Gadgetopia.)
C. David Gammel, CAE 
Good thing MS empowered us to be able to turn that off!
My estimate is that IE7 will have numerous security holes right off the bat and ultimately won’t be protecting the users anymore than IE6 currently is. Your best bet is to use something that isn’t so tightly bound to the OS like Opera or Firefox.