Glimpse of the Future
For what it's worth, here's my interpretation of the invite and likely announcements:
1) The half-revealed Lion simply means that this event will offer a preview/glimpse of some of the features of the next OS X release, which will of course be called Lion.
2) The aluminum and the mention of Mac in the tagline, plus what we know of typical Apple refresh cycles, means this event will focus on Mac hardware (laptop) lineup.
3) We will see the intro of the new MBA. As speculated, this will likely include a new physical design (even thinner), I'd suggest SSD as default, possibly built-in 3G. I feel like the original MBA was Apple's "proof of concept" machine. A lot of the technologies and design techniques which were first introduced in the MBA subsequently were used in the full range of laptops. Hence, Apple tries out this stuff on a small segment of ulta-premium consumers before adapting them for the (relatively) more mass-market segments of its customer base.
4) I think the "Back to the Mac" is both a subtle acknowledgement of/response to the criticsm that Apple has focused too much on iOS/idevices AND a reference to integration of Lion with cloud computing. Think about the "back to MY mac" feature in Leopard - access and control your mac and your data from anywhere (provided you're within the Apple ecosystem). And think about the new Apple data center, move to cloud-based applications (inc itunes). And then think about the recent Apple hiring for developers to work on revolutionary OS architectures which are related to HTTP/large web.
I would speculate we'll see some sort of very deep integration between the Lion OS and HTTP/cloud, and that this event will be the first preview of some of the features that will enable.
I don't know what those features will be, but I am sure this very deep integration of the OS and the web is the next major development from Apple in terms of their technical strategy.