It seems that Cumulative Patch for Internet Explorer (822925), issued on August 20, 2003, changes IE behaviour as describe David Bures in his czech weblog (don't worry about language, David describe this problem also in English) - http://www.websky.cz/ShowEntry.aspx?day=20030821&time=113001.
Seems interesting, and I agree that this issue may cause some problems in already written (Intranet) apps that use frames.