Surely you should just be making sure that your site does display properly in IE6 ;)
I guess you could use it to say "I just want you to know that I went through hell to make this site work for you and your ancient browser. I and every other web developer would really appreciate it if you made the effort to update your viewing platform at intervals shorter than 9 years. Thanks"
Jokes aside though, with up to 40% of web users still using IE6 (and many of them not out of choice) it seems a little premature to start taking a stand. And if any site ever pops up an alert that is not absolutely essential I would usually just leave. That might be just me though :)