Apple Developer Connection sent out an email about QuickTime for Windows yesterday (see previous coverage). It read in part:
IMPORTANT INFORMATION FOR WEB DEVELOPERS & WEBMASTERS
Microsoft Internet Explorer for Windows, versions 5.5 SP2 and 6.0 (soon to be released) no longer support Netscape-style plug-ins, such as the plug-in installed as part of QuickTime 5.0.2 and earlier versions. Consequently, Windows customers using these versions of Explorer who visit your site may be unable to view QuickTime content in the browser. To mitigate this problem and to ensure a positive experience for your Web site visitors, you should add to your site the HTML code that makes available the new ActiveX control for QuickTime, as described below.
Until recently, the plug-in installed as part of QuickTime worked for both Netscape browsers and Microsoft Internet Explorer on both Windows and Mac OS. Now Windows users who try to play a QuickTime movie in Internet Explorer version 5.5 SP2 or later will encounter the "broken plug-in icon" until they install the new QuickTime ActiveX control from Apple in addition to the QuickTime plug-in. Users of other browsers on either Windows or Mac OS are unaffected--they can continue to use the plug-in installed with QuickTime and do not need to get the new QuickTime ActiveX control.
Once users have the new ActiveX control installed, they are also unaffected; they can also view existing pages.
The full text has been posted to KnowledgeBase article 60883: Windows Internet Explorer 5.5 SP2 and 6.0 with QuickTime.