IE9 is not updating Silverlight Xap file?

The ‘beauty of the web’ is giving me problems with Silverlight 4 applications. I am publishing a new version of the application to the webserver, but IE9 is refusing to download (or even check?) for the new Xap file.

Note the version number:

Looking at the Fiddler log, it is indeed not showing the request for the xap file:

Although that the IE9 Developer Tool log does show a request and a 304 response for the xap file:

However, when I look at the IIS log files on the server, there is no such request being logged.

When I do the same exercise using Google Chrome:

We get the correct version number:

We also see the request for the xap file in Fiddler:

And there is a log entry in the IIS logs:
2011-03-18 11:19:33 217.21.176.222 GET /acos/ClientBin/Borealis.Acos.xap - 80 Euri\Rudi.Larno 217.21.176.248 Mozilla/5.0+(Windows;+U;+Windows+NT+6.1;+en-US)+AppleWebKit/534.16+(KHTML,+like+Gecko)+Chrome/10.0.648.151+Safari/534.16 304 0 0 217

Conclusion: IE9 FAIL