Oh man, this one was a doozy. After over an hour on the phone with RIM (retrying everything we had done prior, mind you), the RIM agent mentioned something about attachment redirecting offhandely and that sparked the user's memory about an IT policy they have at her company.
Messages are sent to the Inbox and synched to the handheld at the same time. In summary, when someone views an attachment, the handheld looks at the inbox/mail file to retreive it. Basically, this user had a rule in exchange that sent ALL her mail to her local hardrive because she did not want to fill up her 50 MB mail quota.
So mail goes to mail file -> Handheld synchs with mail file and gets email -> Email on server is then MOVED to local HDD.
When she went to click "Full Content" or "Table of Contents," on the attachment, she was retreiving a popup that said "Could not retieve more," because the handheld was looking for the attachment in a location that didn't contain the mail file because it was located on her local HDD. But because mail is synched right away with her handheld, she was able to read the email, but not open attachments.
What a headache!