What is the pr-requisites to learn Ajax?
The Ability to read and write basic Javascript, or at least know where to look to hack things together.
How hard is it to learn Ajax? And is it worth the time, or by the time I learn it, it will become obsolete?
It's really not difficult, at all, to get something up and running. Just as an example, I did a couple presentations a couple months back to my co-workers explaining what Ajax is and how to use it. Within an hour presentation, they had everything they needed to go out and make Ajax enabled apps.
Like anything, though, there are good and bad practices. For something very simple, you don't need to invest a lot of time. For more complex Ajax applications, you probably want to spend some more time learning what is "good" and "bad". But I'm sure you do the same with any technology.
Will it become obsolete? I guess I don't see that happening in the very near future. Ajax is not usually necessary, but it can be a great way to make a web application more responsive and more useful to your end-user. Anything that makes life easier for your end-user is a good thing. Ajax is not the answer to all problems, but it does have a nice niche that it fills.