That is, it should be showing up now IF you have an iPhone 3GS, iPhone 3G (some features won't work, like multitasking), or a late-model iPod Touch. 1st Generation iPhones and iPod Toucheses are forever OS 3.
I've been using the developer build for a week or two, and it's very nice. The multitasking UI (double-tap on the home button to bring up the hidden dock) takes some getting used to, and the folders aren't as pretty as the app icons they replace, but it's all very functional and practical once you adapt to it. Third-party app updates that take advantage of the new features are coming fast and furious.
The new features make things feel much faster, and the system overall feels noticeably faster on the same hardware. It's nice that Apple's focus isn't solely on adding new features; they also made tons of tweaks—both visible and under-the-hood—that result in the whole thing feeling incrementally more polished.
I particularly like the rotation lock, as I work mostly lying down.
Ars Technica has a good walkthrough that covers most all the new features, big and small, as does this awkwardly named blog.
