PalmSource Ships Palm OS 6
grndslm notes: "PalmSource has indeed shipped Palm OS 6, the next major release of its operating system, to its licensees. The company has posted a press release this morning, PalmSource CEO David Nagel will also be presenting at the Needham Growth Conference today as well."
First, the reason you don't see a "Buy it now!" or similar button is because this is not a release to the general public, but instead just to licenced developers such as Sony, Handspring, PalmOne, and Kyocera. However, as far as I know the answer to your question is "maybe." Upgrading to Palm OS 6 depends on three factors:
Hope that helps some. I would hazard a guess and say that ARM-based devices from Palm (Tungsten|T, Tungsten|C, Zire 71, etc) will be upgradable, and that Palm will provide such an upgrade, simply because they've done so in the past. Couldn't say for other Handspring or other vendors.