A big thing I have been noticing that have been popping up in a lot of sites that I have been visiting is the user alert that they are using a browser that the site cannot be viewed properly, or that they are using an unsafe browser.
Though the message is out of courtesy, I have been noticing an alarming number of sites preventing users from even viewing a website because of the user’s browser choice.
While I agree IE6 is not safe, I believe it is best when addressing the matter to be cordial and unobtrusive on a website. Create a page arguing your case and state facts to tell your visitor why they should convert. The link should be in the sidebar. You can either link it with an image or just leave it in text.
It should always be a matter of choice when surfacing. Forcing your user through annoying pop-ups, or text that distorts the layout can discourage your user from not returning to your site. Think about it.