Quick question about customs - will we go through US customs in Jamaica (Montego Bay) at departure, or when we arrive in Philly? I just realized we have a connection on the tighter side in Philly and want to make sure we have enough time....thanks!

You go through US customs in the US.

Preclearance exists at most major Canadian airports, providing convenience to travellers from those cities to the U.S. Arrangements also exist with some airports in Bermuda, Bahamas, Aruba and 2 major airports in Ireland.

I was answering his specific question.

The beauty of public forums - others read the threads. Perhaps I stopped another similar question about Costa Rica....

Right! We love adding information totally useless for the OP. But guess what? The OP can stop reading after the first reply! Yes, that's the beauty of it. Nobody forces the OP to continue reading.

BTW, for DUB and SNN in Ireland, you only preclear US immigration. Still have to go through the customs hall after arrving in the US and hand it the declaration (or got sent to secondary screening).

Thanks for the info. The reason I asked was that leaving the Bahamas you go through customs there.
Another question - will an hour & 45 minutes be enough to make our connection (given that this is USair) or should I make a hotel reservation at PHL just in case?

If everything goes smoothly, an hour and 45 should be plenty. Have you chosen your seats yet? If I have to go through customs I try to sit near the front so I can zip out of the plane before the line starts to really build up.

PHL is relatively small and easy to navigate, IMO. Customs usually runs smoothly. However, if there's a delay, or bad weather, or whatever, you could run into a problem with being delayed in the air or on the tarmac.