When enabled InPrivate Blocking will block a resource when a user's browser has seen the resource referenced across 10 different sites. So for example, with InPrivate Blocking on, jQuery sourced from Google will be blocked by IE8 after the browser sees that file sourced by more than 10 sites.

You can read more about it at http://blogs.msdn.com/ie/archive/2008/08/25/ie8-and-privacy.aspx under the "InPrivate Blocking" section.

You could obviously host everything that you need yourself to get around this, if you are pulling things like jQuery from Google.

If you host a file that is sourced by others, with some server side URL rewriting, you could allow for random strings to be added to the URL that would be ignored by your server. This would allow others sourcing from you to create unique URLs for the file so that a user's IE8 browser won't see the same URL sourced across multiple sites.

Any other ideas?