You can purchase a Pixi "contract free" for $400, but would still have to use it on Sprint's network. I'm guessing you're looking a SIM based Pixi that's unlocked. Pixi is CDMA (Sprint) at this point. If a GSM model becomes available, I'm sure there will be unlocked versions either from Palm or eBay.
Yes, I bought an AT&T palm pixi from ebay didn't say it was unlocked when I got it. However, I put a tmobile sim and an Orange sim in and they both worked so yes anything is unlockable that has a sim card slot.