RoR does have the potential to seriously challenge J2EE. Remember, it is only 3 years old or so and needs lots more work and
testing to be acceptable to companies now relying on EE.
I seriously doubt companies would trash all the work and money and jump into rails or anything else. If RoR does eventually take over the enterprise space(not even close to a certain thing), Java will still be around for a long, long, long time. At worst, there will be thousands and thousands of jobs to maintain all this Java code, and EE will still be used. After all, J2EE isn't the only enterprise framwork(s) being used. Fortran is still used today in specialized areas, and there is still a lot of legacy Fortran to maintain.
Is it worth it? Yes and no, depending on your goals. If you are interested in creating small-ish web sites, I think a person would be nuts to use anything but Rails.
Servlets is overkill for small projects and PHP is a security mess and encourages poorly written and designed web pages. I don't know enough about Zope to comment.
If you want to get a enterprise level job, you would be nuts to not learn J2EE.
Then again, you would be nuts to only learn one language/framework. I haven't seen statistics, but it would not surprise me to learn that most programming projects use multiple languages.