Checking the UPS website this evening, I found that they made a delivery attempt in the afternoon. Didn't see a delivery notice though so I checked around and found it at my neighbor's! Argh. I guess they made a mistake or the package was misaddressed. I'll have to check with the sender and see if I can get this straightened out.
And here's the other problem... I thought I'd already given them blan... uhh... general authorization to leave all packages at my door without requiring a signature. I guess my neighbor hasn't given such authorization so if it is indeed addressed to my neighbor, they aren't going to leave it there.