This is far from new technology - multistream and varistream were long available from Real and Microsoft. There are subtle differences, but the effect is the same. (These shouldn't be confused with variable bit rate - VBR).
Most technology available now detects the bandwidth the user has available and delivers one of a number of pre-encoded streams - or dynamically transcodes a stream accordingly. This is usually reliable for domestic broadband, but the situation with mobile is different - data rates can vary tremendously even within a cell due to 'bandwidth slicing' - i.e. sharing the bandwidth amongst the active users in a cell.
So, this is pragmatism by Apple, which has long had a strange relationship with internet TV. But adoption of the http protocol is a dig back at Adobe, who famously dissed Steve Jobs some years back, and is a threat to Flash video's very expensive server products.
The reality is that most users cannot tell if they're seeing streaming or progressive downloads, and don't particularly care.
Another by-product of adaptive streaming is that it can be used to make downloading that much more difficult.
However, it does make media preparation much more complex and adds yet another level of complexity to the web video production chain.
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