I went and looked at the text of this patent only to find myself knee-deep in legalese and vaguery. It appears at first sight that anyone who uses an interactive browser plug-in is at risk of falling foul of this patent, but it's actually intended to cover cases where the program providing the plugged-in content is running server-side and communicating with the user via IPC (inter-process communication). I'm not sure which bit of IE is meant to infringe that: fingers have been pointed at ActiveX but that's a rather large haystack in which to find the offending needle. And among all those `said servers', `said clients' and `said objects' I don't know how non-specific the text of the patent actually is in legal terms.
Reports of what Microsoft will do next range from `completely rewrite IE' to `disable a small part of ActiveX' but I can't help hoping it teaches all those people who have web sites `best viewed with Internet Explorer' a lesson or two.
You've got to laugh at MS having another misfortune in the courts, but that doesn't actually justify the action. In any case there's nothing saying Netscape/Mozilla and other browsers that use plugins are immune from future action.