Never, never, NEVER click on a link or button in an email asking for personal or financial information -- that's a sure way to become a victim of a phishing scam. If you receive a note like this, log into your MobileMe account and update your billing info directly, if it really needs to be updated. Do NOT click on the button.
A couple quick ways to see where a link or button in an email is going to take you are to hover your mouse over a link to see the real URL, or to right-click a button and select properties from the menu to see what URL is embedded in the button. Be careful out there, folks -- there are a lot of unscrupulous people who would love to take all of your money.
A tip of the hat to Noah for supplying the screenshot!