In contrast to the original poster's experiences, I was severly hassled by U.S. Customs for bringing back a bottle of 100 Tylenol #1's from Canada. The legal limit, of which I was unaware, is 50 tablets. Eventually they let me go with a stern warning, but also allowed me to keep my 100 tablets.

For those who aren't already aware, Tylenol #1 (which contains significantly less codeine than Tylenol #3) is sold over-the-counter in Canada. However, you won't find it on the shelf... you must ask the pharmacist for it.