Keep Circulating the Tapes: The series have been off the air for ages, and many of the episodes were not released to DVD. And then those DVDs are no longer manufactured pretty hard to track down new (Amazon has a few new condition copies of the volumes scattered around the marketplace, but good luck finding all of the volumes in new condition). Also applies to the Second season theme song which is only aired a few times in the series' entire run. And let's not get started about the episodes that have not been exported out of Japan in the first place...
No Export for You: The later seasons of the TV series and the spinoff TV series in it's entirety. Also, all the movies and half of the OVAs, of which some were tie-ins with the games.
The first Game Boy Color game and the first DS game. Also, the fourth game, Rainbow Rescue, received a release in Europe and Japan, but not in the US.
The Sega Pico and Advance Pico Beena titles. Expected, given that the latter was a straight case of No Export For You and the former pretty much died an early death outside Japan.
The official kiddie ride by Ban-Dai/Banpresto is Japan-only and never exported. Expected, given that it came out when Cartoon Network was almost done screwing up the show elsewhere.
Introducing TwitHamtaro(translation), yet another Tweeter app for the iPhone, with a Hamtaro skin! Want it? Sorry, it's only available on the Japanese iTunes Store, and thus only to those lucky enough to have a Japanese address and credit card!
Playing Against Type: You know the awesome Scott McNeil? Try to imagine him as a shy teacher who has a crippling fear of chickens. Yeah, he's Mr. Yoshi in the show.
Screwed by the Network: The second opening theme song only aired a few times (counting reruns) in the US, by accident. Yeah, Cartoon Network hates it for some reason.
Writer Revolt: Hamtaro's placement on Toonami lead to a number of jokes by Williams Street.