A couple of more caveats:
If your grid pre-screens content -- say, for example, not allowing users to upload stuff until a grid managers checks that it's not infringing -- you may lose your "safe harbor" status.
That's because a copyright owner can claim that you knew all about the infringing content they found on your grid -- because you screen everything ahead of time.
Similarly, if you step in and try to mediate copyright disputes between users, then instead of suing each other if they're unhappy -- which would be the case under the DMCA -- they could turn around and sue you.
The DMCA specifically states that your actions should be automatic -- you automatically take down content once a take-down notice comes in, you automatically put it back up again if there's a counter-claim and no legal action. If either of the two parties want to take it further, they can take it to the courts -- where they go after one another, not you.
If you step in and take one side or another, then you stand to become a party to the dispute. You're no longer a neutral platform. And you lose your safe harbor.
That doesn't mean you can't get proactive, though. The DMCA specifically allows for "technical means" to sniff out infringing content. This means this like watermarks and digital signatures on movies and music, but if someone were to come up with something similar for virtual content and would easily allow copyright holders to discover infringement, then it's okay to use this technology (and depending on the situation, and if the technology isn't too much of a burden to you, you might actually have to use this technology to maintain safe harbor)