The importance of platform independence can't be understated for small, cash- or time-strapped developers. In theory, it means that you can write a program once and have it work across any device that supports the AIR standard, saving a lot of efforts and enabling you to sell to markets that might have otherwise gone untapped.
Moreover, AIR is a close cousin of the entrenched Adobe Flash standard: AIR basically exists to run Flash content outside the browser as a full-fledged. That's why AIR support for the Flash-less iPhone is such a big deal; Flash developers finally have a way to package their applications for Apple's device, even if it does have to go through the iTunes App Store.
AIR has already proven its viability on the desktop with applications like the popular Twhirl Twitter client, which I use when I run Windows and fellow blogger Dave Courbanou uses on his Mac. If AIR really catches on, it's entirely feasible that you might be able to use the same exact executable on my desktop as my portable devices.
Java, despite an undeniable amount of success, never truly and fully lived up to its promise of independence from the shackles of OS exclusivity. Adobe isn't an upstart by any stretch of the word, but it remains to be seen if they have the technical acumen to make it work and the business drive to really attract ISVs to their platform.