Ask your customer this: are they willing to upgrade to Vista? If they say yes, then don't support IE6. Your target customers are the people who goes "whoa! vista. drool". They're also the kind of people who want the fastest and most powerful computer.
If your customer goes, "huh? what's vista? I want my screensaver of cats back please", then you need to support IE6.
In short: if they have Vista, then they don't have IE6.
The irony is: for web developers to finally get rid of IE6 and its legacy, they have to promote Vista or hope that Vista will be successful.