The depressing thing about discussions like this is how much misinformation gets spread around. Here are a few examples:
There is one thing about Java which can be a problem for scalability, and that is the proprietary nature of many of the server environments people run it on. If you buy a commercial J2EE server, the vendor will promise you incredibly scalable session data storage and database caching, but they will typically not tell you how they do it. When you run into trouble, you have no knobs to twiddle and no alternate options because the whole thing is closed to you. This is why I would recommend that people wanting to use a Java solution should look at the open source tools instead, especially the new lightweight ones.
The most unfortunate thing about this PHP/Java discussion is that people still aren't talking about the things that actually do make a system scalable, like ways of limiting the resources that each active user on the site takes up, and ways of handling overload gracefully.
Okay, I guess I had a little more to say about this than I thought. I'm going to stop here for now. Maybe some of this will filter into my OSCON talk.