The V5.5 isn't available anywhere yet. They're supposed to come out, in Asia & Oceania (Australia) in October 2008.
However, from the pictures, they're identical to what's being sold in the USA as the V5 refresh model, which we know as the V5 Celebrity. From this, I don't expect to ever see any Tamagotchi sold as a "V5.5" in the USA. Note: the V5 Celebrity has been available since July 2008, but even now, not all stores have them yet. You can tell them apart from the regular V5 model by the package color. The V5 Celebrity has a mostly dark blue / purple package. Here's a link to pictures of both types on ebay so you can see what I mean. Regular (orange at top, yellow at bottom): click here, Celebrity (purple at top and bottom): click here