Under IE6, make it at least show something. A page for FF3 that just dies on IE6 just looks bad, like you didn't plan well. If you don't support IE6 at all, make sure the user knows it is deliberate by showing a special page advising them where to go.
If you are expecting corporate visitors, it has to work under IE6 even if only a simplified version. If not, you can drop IE6 entirely if you handle it well as described above.
The time is nowhere near ready to consider dropping IE7 though. I'd expect this is the default browser on XP, which is the most prevalent OS.
