The Flipnote Studio site has recently received an English translation (albeit all the comments and videos are in Japanese) which is pointing to a release in North America soon.

Using Flipnote Studio, users will be able to create their own animation and upload their creations to Hatena. Users can comment on the creations (with text from a computer or writing it out on the DSi), and flag inappropriate penis animations that are sure to flood the system sooner or later.

(Note: Wikipedia is saying without citation that it's already out in NA and Europe and Australia get it Friday with citation).

Edit: Flipnote Studio is actually available in North America right now!

Source