It has existed since years ago. Gekkoscience do have a USB miner that outputs 20+ GH or so.
Ah! I wasn't aware of that. Thanks for the pointer.
It looks like with some effort, a USB 2.0 dongle could reach 0.5 TH/s (consuming 2.5 watts)
That is, 1.8 KWH / month (assuming 24h/24h operations - unrealistic!), that is roughly 4 USD energy costs per month - less than your bank fees!
If such a dongle was available, it is likely that many bitcoin (and other alts!) users would purchase and use it, as it guanrantees their own security (ie protection against 51% attacks / other forms or attacks...), this would decentralize mining effectively.
Let us suppose that such a dongle would sell for 20 USD, profit 5 USD.
When motherboard manufacturers will realize that they can get enter this market easily by providing it on board, at cost 10 USD, profit 2 USD, they will do...
So the question is: how to lower the barrier of entry to mining and make it affordable for the masses.
If the (current) bitcoin protocol has been foreseen to cope with tenths of millions of miners, that should work and is only a question of time.