This is not a altogether new bug. I've had previous encounters with missing 
attachments from recipients of outlook emails. I'm paraphrasing memory but I 
when I last hit this a few years ago I found two KB articles pertaining to 
dropped attachments- one said to not use Word as the email editor, the other 
said don't send in plain text. (scout's honor)

And from the Exchange 2007 deployment doc:

"The Exchange 2007 MIME parsing engine, developed and evolved through 
many years of MIME handling exposure, is likely the most trusted and robust 
MIME engine in the industry".