Slashdot has a discussion based on a number of amici for MGM v. Grokster. Having just done some writing on the topic, I think the case should focus on two issues.
That Grokster has substantial noninfringing uses (as per Sony, 464 U.S. 417 (1984)) will probably come into controversy, but I think should definitely be accepted. It should also be accepted that Grokster has no ability to regulate infringement on their system (they could close their doors and shut down every computer and it would have no effect). They cannot ban users or cancel downloads. These are the two tests Sony set up to qualify for protection from contributory infringement, and Grokster should pass them both.
So what comes into play is the 7th Circuit's Aimster, 334 F.3d 643 (7th Cir. 2003) holding that Aimster had willfully blinded itself to infringement and therefore it didn't matter that they were unable to regulate it. The 9th Circuit gave no real consideration to this issue in Grokster, 380 F.3d 1154 (9th Cir. 2004). I think Posner's willful blindness approach makes a lot of sense and is a necessary addition to/interpretation of Sony. Without it, Sony is easily gamed, and stands a strong possibility of being overturned. So the question here would be whether or not Grokster has valid reasons to be designed in the manner it is (as compared to Napster, which could exert control over individual users and transactions to regulate infringement), or if it's simply an effort to bypass Napster and get under Sony.
The other issue is whether Napster, 239 F.3d 1004, 1021 (9th Cir. 2001) (and to an extent Aimster as well), knowing what we know now, was wrongly decided. One of the arguments advanced by the CS professors in the amici mentioned on /. is that copyright screening is very difficult in these sorts of systems and is effectively crippling to them. Sony dictates that shutting down the service/manufacturer altogether is not an acceptable remedy if the service/item has substantial noninfringing uses. Thus the assumption that Napster could control infringement simply because they do have the ability to ban individual users may be mistaken. If it is only an illusion of control, then Napster should have been protected under Sony, and so should Aimster and Grokster and all the rest of them.
If the Court does take up the willful blindness test they'll probably have to remand, because as far as I can tell (I only had access to the 9th Circuit's decision, not the district court or any of the briefs), the issue of Grokster's design never came up in any meaningful way and will need to be argued. I think their chances on that issue are fair. There are benefits to be gained from the supernode structure: lower administrative costs (no need for massive servers and bandwidth), no single point of failure for the network, possible performance benefits. I don't know how strong these benefits really are nor where the bar is set to impute intent on willful blindness.
The technical argument is interesting. I think there can be no argument but that it's very difficult to screen for copyrighted material on a p2p system. But whether it is cost-prohibitive to the point of being impossible, I really can't say. It might be a function of how much money you have to throw at it, which only raises more complications. Would p2p be liable for contributory infringement if it's run by a commercial entity, but not if it's run on a non-profit basis? I'm not sure the Court will want to open this can of worms.
That, of course, gets to another point I think is very interesting in this case. To an extent it doesn't really matter that much. There are a fair number of open source p2p projects. Even if Grokster and every other commercially based p2p system gets creamed by contributory liability, it will probably not even put a dent in p2p usage. p2p is relatively easy to do. An individual programmer can slap a fairly sophisticated p2p system together (as Shawn Fanning and Bram Cohen can attest). And once they're up and running, they're very difficult to take down. As is mentioned in Grokster, there is nothing Grokster the company can do to shut down Grokster the p2p system at this point. It's beyond their control. Although both sides will make grand claims to contrary, ultimately, I don't think this case means much to p2p. Its relevance will be purely in terms of how Sony is interpreted from here on out.