The iTunes 10.5.1, available for both Mac and Windows users, addresses a flaw that lets a man-in-the-middle attacker offer harmful software that appears to originate from Apple.
From Apple's advisory:
iTunes periodically checks for software updates using an HTTP request to Apple. This request may cause iTunes to indicate that an update is available. If Apple Software Update for Windows is not installed, clicking the Download iTunes button may open the URL from the HTTP response in the user's default browser. This issue has been mitigated by using a secured connection when checking for available updates. For OS X systems, the user's default browser is not used because Apple Software Update is included with OS X, however this change adds additional defense-in-depth.
iTunes 10.5.1 can be downloaded directly from Apple's web site.