I am having the same problem on one of my computers. When I send or forward an email, the first letter in the subject line is lopped off. It is not a typing error. At present (when I remember), I place a space before my subject. I was out ot town for several days leading up to the 5th of May, 2008 but that was the date of the first occurrence. The last Outlook updates on my machine occurred two weeks earlier.
I have only come across a handful of people with the same problem but noone responded with a viable solution to the individuals who requested them. I hope someone in here can help.
Also, it does not appear to be related to any virus/spyware/firewall programs. Further, apparently some people had the same issue with Outlook 2003.