I have been experiencing slow iTunes downloads as well, and this article showed up in my RSS reader this morning:
http://www.appleinsider.com/articles/10/12/20/appletv_itunes_downloads_slowed_by_googledns.html
It seems to only be speculation at this time (nothing confirmed by Apple), but using an alternate DNS service such as Google DNS or OpenDNS might be to blame.
I am indeed using Google DNS and reverting back to my ISP's DNS as I write this post, so I can't comment on it yet but I'll report back to this thread in a few days with my results (improvements or status quo in iTunes download speed).
If anybody following this thread is using an alternate DNS service, it would be helpful to get your results as well to see if it is indeed the cause of slow downloads.
Pascal