Looks like O'Reilly and CMP recognize a PR problem when they see one coming down the pike - they've called the dogs off of the IT@Cork conference. They still insist that they own the "Web 2.0" moniker though, and state that it's like the term "LinuxWorld" - a stock use of a trademark:

O'Reilly and CMP co-produce the Web 2.0 conference. "Web 2.0" was coined when we were brainstorming the concept for the first conference in 2003. As noted in the letter to IT@Cork (sent from CMP's attorney, but with our knowledge and agreement), "CMP has a pending application for registration of Web 2.0 as a service mark, for arranging and conducting live events, namely trade shows, expositions, business conferences and educational conferences in various fields of computers and information technology." To protect the brand we've established with our two Web 2.0 Conferences, we're taking steps to register "Web 2.0" as our service mark, for conferences. It's a pretty standard business practice. Just as O'Reilly couldn't decide to launch a LinuxWorld conference, other event producers can't use "Web 2.0 Conference," the name of our event. In this case, the problem is that it@cork's conference title includes our service mark "Web 2.0," which the law says we must take "reasonable steps" to protect. We've also contacted another group that has announced a "Web 2.0 Conference" in Washington, DC this September.

Here's the problem - the term "Web 2.0" is way, way too widespread for them to claim ownership. Here's a Google search on it - see what I mean? The law may well be on their side, but common sense very much isn't. If they think they're being harmed when others use that term, then they need an appointment with a cluestick.