In lieu of an upgradeable demo (they're not allowed on the App Store) the original Myst's iPhone and iPod Touch incarnation now has a "free version." Myst FREE is a glorified demo that allows players to explore Myst Island, but entering any of the "Ages" will require a full download and $5 charge -- a $1 price drop from when the title was first announced.
At 123MB, the demo's download is substantial. But that's nothing compared to the full game's 700MB footprint ... and it still beats having to hook up a CD-ROM drive to your iPhone.
iTunes - Download Myst FREE at the iTunes App Store