Swarmcast has released a free iTunes download accelerator, caller Autobahn, which claims to accelerate iTunes downloads. The software is for Microsoft Windows only.
Swarmcast claims an increase in download speeds of up to 90% (10X improvement) by using what they call multi-source streaming. This seems similar to bittorrent in theory except it’s not p2p, instead all the sources are servers.
Currently, when I download from iTunes I typically download at the maximum speed of my DSL connection so I’m not sure this would help in these cases. Since it’s currently Windows only and I don’t run iTunes on Windows I haven’t tried it myself. They are working on a Mac version and I’ll give it a try when that comes out. No date for the Mac version has been published.