There are a number of public network attacker threat feeds available, the most well know of which, AFAIK, is the Internet Storm Center's DShield system. I know a few network operators, including at least one on this list, also run private versions of the DShield system.

Are there many others?

Do any or most network operators have some sort of private current block list that gets pushed out to routers and or firewalls/traffic shapers in real time?

I'm the CTO and founder of ThreatSTOP (www.threatstop.com), and we're currently propagating the DShield, and some other, block lists for use in firewalls. I'm interested in gathering additional threat information, and serving additional communities.

Is there any interest in a collaborative platform where anonymized candidates for blocking would be submitted by a trusted group, and then propagated out to the whole group?

I'd be happy to collect responses anonymously and submit a summary back to the list, if people don't want to open this up on the list.