In fact, the code states: "Whoever, without authority, opens, or destroys any mail or package of newspapers not directed to him, shall be fined under this title or imprisoned not more than one year, or both." 18 USC 1703
So far, I've only had to go to the post office 1 out of ~10 times, and that because the postal company said they were contractually obligated to get my signature. (So I don't get Dell shipped to my unit any longer.) It's striking how that one line changed everything for me.
That said, re. startling's comment elsewhere in this thread, I don't think lack of buzzing means they didn't try -- they often have to drop off more than one person's package in my building, and I never see delivery folks waiting by the door buzzing every individual's number ;-)
What made it even more annoying was that many times that company would leave packages at the front office when I was home. They would not even try to deliver to my apartment. They would just go straight to the front office.
Anymore, if I get to choose then I choose FedEx, because it's very easy to have the delivery re-routed to a FedEx/Kinkos store, and pick it up there. UPS re-routing has been impossible for me.