For a few years now, Microsoft's Internet Explorer advertising strategy has been to say "Internet Explorer used to suck, we know!" . This is sometimes cute — the IE6 funeral was funny enough — but the premise never quite worked. The implicit message is that Microsoft understands something that is very obvious to everyone else, and that this is a reason to return to its new and now slightly better product. It's like a buddy who's always a jerk at parties regretfully chuckling about the time he fell into your wedding cake. Guy: Thanks for apologizing, but you are no longer on the list. You will not return to the list.
Maybe this works better: Advertising for a web browser is like advertising for tap water. If the water that comes out of your faucet is potable and tastes like water, you stop thinking about it. It becomes invisible. You drink it.
If it gets you sick, or gets your mother sick, or tastes like rust, you switch to bottled water. Likewise, a good browser is a browser you never have to think about. That's all it is.
Internet Explorer 6 was not potable. It was not invisible. It gave a bunch of people terrible diarrhea and those people will probably never give Internet Explorer another chance.
Microsoft: Let IE 9 disappear. Let those who have forgotten what happened drink your water, which is now indeed safe. People will stop leaving if you stop giving them reasons to. But for god's sake, do not remind people about the time you gave them diarrhea.