I wrote an update to my original blog on the EHLO site regarding Delegate and DL problems in multi-domain environments. This Outlook update helps address the delegate issues that you were seeing previously that caused a lot of folks to redesign their AD infrastructure to work around some of the pitfalls associated with this.

Check it out at http://msexchangeteam.com/archive/2008/01/24/447928.aspx

Update!!
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This change has now been ported to Outlook 2007 and the associated KB Article is http://support.microsoft.com/default.aspx?scid=kb;EN-US;950794