Apparently, the other differences between an American P1 and an American P2 are:
1. The background, as ipickee said.
2. The characters. P1 has Bebitchi/Babytchi, Marutchi, Tamatchi, Kuchitamatchi, Mametchi, Ginjirotchi, Masukutchi/Masktchi, Kuchipatchi, Nyorotchi, Tarakotchi and Gaijintchi/Bill. P2 has Shirobebitchi/Shirobabytchi, Tonmarutchi, Tongaritchi, Hashitamatchi, Mimitchi, Pochitchi, Zukitchi, Hashizotchi/Hashizoutchi, Takotchi, Kusatchi and Zatchi.
3. On the feeding menu, instead of displaying the words Meal and Snack, you've got pictures of the foods.
4. The games are different. On P1, you have a guessing game where you have to guess which way the Tamagotchi's gonna look. On P2, you have a guessing game where you have to guess whether your Tamagotchi's number is higher or lower than yours.
5. The medicine icon's different. On P1, the medication icon was a syringe. On P2, the medication icon was changed to a medicine bottle and spoon. I remember reading somewhere that this is because kids felt guilty about giving their Tamagotchi injections, though I'm not quite sure if this is true.
6. The designs.