-shrug-

‘you don’t need to either download or pay for an application to view HTTP video streams, since in theory you can view it in a browser’

Not correct. They’re just using HTTP to push the data over. You still need Quicktime X to view it.
This isn’t anything new, most flash videos work in this way, aside from using multiple streams.
In theory, with enough Javascript and the right callbacks you could do this in the HTML 5 video element.

‘Using HTTP also allows content to be broken down into viewable chunks so the provider can send the most optimized chunk to viewers at any given time.’

This has nothing to do with using HTTP, this can be done over any transport protocol you care to choose. The downside to HTTP is that for real-time streaming you get lag if you can’t download it quick enough, or have high error rates.

What Apple have created is this:
1) A media splitter that takes a file and re-encodes it in around 10 second chunks at various bitrates. It also produces an index file saying what filenames are which parts and which bitrates.
2) Got Quicktime to download a chunk and if it takes too long, go for a lower bitrate one for the next, and if it came down easily, a higher bitrate one.

The most likely reason for the higher quality over the
SlingPlayer Mobile is that the re-encoding is done offline, so you can do better analysis of the stream and make the best use of your bitrate.

This in no way requires an OS update, just a Quicktime update.

It’s a clever idea, but not a new one (both RealPlayer and Windows Media Player support this already) and it wouldn’t be too hard to replicate this in Flash/Silverlight in an afternoon.