Where do we go through customs on the way home?

Customs and Immigration will be in Seattle. When changing BA flights at Heathrow you will remain at Terminal 5. You will, however, also go through a passport check station, like immigration but it is not for the US, obviously, and then security in Heathrow when you change flights.

At your departure airport in Italy the airline will check your passport to verify you are the person on the ticket. You will then go through emmigration (passport check). If you are a non EU/Schengen citizen your passport will be stamped for leaving the Schengen Area at this point. There is no exit customs.
At Heathrow you are in transit, the airline will check your ID again, as Wray describes, but nothing else.
You do not encounter US officialdom until you land.

At Heathrow, between your BA flights and prior to approaching security to re-enter the secure area of Terminal 5 you go through passport control with British officials, not airline officials. They behave like Emmigration/Immigration individuals but they know you are re-entering Terminal 5. I don't know what they are officially called, but they are there and their behavior is the same, and there is often a long line to get through them. Those who have never transferred between a flight into and out of Terminal 5 deny their existence, but they are there and it is another step with British officials.

Go to this page:

Heathrow Flight Connections

fill in your flight information and you will get step by step procedures.

Make sure you follow the "Flight Connections" sign and don't just follow everyone to immigration/passport control. You shouldn't be going through passport control if you do it right.

Excellent info, everyone, thanks! (9 days and counting... we're as excited as kids at Eid/Hannukah/Christmas!)

It hasn't happened yet but, Heathrow is one of the airports where US Department of Homeland Security is considering placing a customs pre-clearance site. They already have such a site at the Dublin Airport - it allows you to take care of the customs while in the airport awaiting departure instead of having to go through it in the end.

Oh that would be sweet, Dawn - any chance you could get them to expedite it, say, have it in place by October 8? We have almost 7 hrs layover and it would be nice if we could take care of that task then and there. Oh well. :)

Flew to Seattle from Heathrow in late June. Immigration and customs was soooooo quick and easy.

Good to know, thanks Charlie. And thanks too, for the reminder MrsEB, about the difference between immigration and customs. Since we're both US citizens with only carry-on luggage and nothing to declare, I imagine that immigration will indeed be the more time-consuming part of it. I appreciate the helpful info.

Do note that if you arrive at Heathrow terminal 5 at either of the B or C satellites, and your onward flight is also at either of these satellites then you do not need to go back to the main terminal (A) to clear security; you can do that at the satellite itself without mingling with the passengers originating at Heathrow. If either of your flight arrives or departs from the main terminal A you do not have this option. Impossible to know in advance as the gates are assigned on the day, so do check on arrival.