The invitation features a cover flow style front with the foremost album cover containing a silhouette iTunes dance holding an iPod (and the other album cover being Beatles' albums).
Speculations say that Apple is going to announce OS X based iPods, probably an iPod Nano 3G with a new form factor (here a mock-up), a full-screen touch iPod (here a mock-up). And probably (as the "The beat goes on" tagline and the album covers suggest) the launch of the Beatles on iTunes.
And last there is also a possibility that some of the new iPods will be able to wirelessly control iTunes, both acting as remote for iTunes and also streaming music stored on a computer to the iPod.
Personally I like to think that the new form factor mock-up is about a new 6G iPod video or maybe even a new model, but not about a 3G Nano, since it is already small, and making it even smaller will make it more difficult to handle (not saying that then all my Nike+ armband and t-shirts will be useless as they were designed for the slim 1G and 2G Nanos).
Already enough speculation and echo chambering for today: let's wait next Wednesday and we'll see what Steve Job is going to announce.