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!!---------------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