http://support.microsoft.com/kb/823232 - says this problem was fixed in
Office XP, but is appearing in 2003 and 2007.

The To Box Is Automatically Populated with Recipients by Lori

Lori
Wed Apr 30 19:40:22 PDT 2008

I have also had several reports on this with Outlook 2003 SP2 and SP3. No resolution as of yet.