I didn’t. Boy, the things you learn from Solosez and mailing lists in general. This, folks, is the wisdom of the crowd, right here:
Someone posted to Solosez the inquiry (no doubt born of desperation) “Can you recall mail once you’ve put it in the US Post Office system?” Personally, I winced for the questioner, and thought “Eep. Sorry, friend.”
Then, however, I read the responses, and learned something really valuable: Form 1509 (PDF).
Here’s a FAQ page on the USPS site that explains it all. The bottom line is this: once it’s delivered, you’re out of luck. But before it gets delivered, you can have the item recalled with a certain procedure.
You learn new things every day. Imagine the reasons you may wish to recall a letter before its delivered. Can’t do it with email! It would be interesting to learn if this postal option is available or even effective in other countries.