BitTorrent uses a lot of expensive bandwidth. ISPs block it to save money and reduce the number of expensive legal complaints that they receive, not because they're evil.
At what stage will Bitcoin be (or construed to be) expensive to the financial services industry? Do we need the technical innovations that will take the network to millions of transactions per day, or will it just be the quantity of money in a given period that will be seen as harmful? I see that Bitcoin would have to become much more prevalent than Bittorrent ever was for it to ever be cast as a threat to the entire monetary system (and it will probably accompany a very different social attitude backdrop, too).
If ISPs did block Bitcoin, they'd be pissing off their customers without getting anything in return for it.
This I'm not sure about, they wouldn't piss all too many off right now, and those they did are far more dependent on their ISP than the ISP are on them.