When I was there, I worked in one of the departments that handled mail. When mail is sent to bethel, a member of the mail room or shipping signs for it no matter who it is addressed to. Depending on who the sender is, (like if its the IRS or something) at times bethel will open mail even if it addressed to an individual. This is legal, part of the dozens and dozens of forms you fill out authorizes them to act as your agent. (They also make you sign one authorizing body disposal).
If it is addressed to "Governing Body" its going to be a secretary, likely an elder, that opens it. I remember signing for all sorts of weird looking packages, I did not open them, but the handwriting on the outside made it look like something some freak like Ted Kazinzke (sp) would send out. The sort of stuff that gives respectable apostates like most of us a bad name.
If it is addressed to an individual member of the GB, I am not 100% sure but I would imagine they have other people open it just in case it has something harmful in it, that it would take out an easily replaceable secretary instead of the big-wigs. This is pretty common with large corportations, especially since 9/11.
I had to go in a few of the GB's offices for some reason or another at various times, and all the mail in the inbox appeared to have been already opened, so I assumed it was by a secretary vetting legit mail from stuff that could be tossed or handled by other parties.