Pad, I am very far away from being an expert in this area, but I can say what I
think will happen ;-)
Major companies which decided to use Spring won't jump into JEE6 at the moment or even in the near future. Firstly it's because enterprise software doesn't like rapid changes and secondly because they learned from the procedural -> objective oriented programming "hurray" change.
The other thing to remember is that the Spring was introduced because of too complexed design of
JEE in it's early versions (up to 1.4). JEE 1.6 with introducing profiles, dependency injection, new
JSF, enhanced JPA, Bean Validation, etc. is not only more easy to use but it also introduces features which only Spring had up to now. In the same time it gives you (theoretically speaking) an ability to change the vendor/implementor of a particular part of your system and not be binded to the Spring Source whole time.
So, the bottom line is that JEE6 is a very interesting platform to build enterprise software and, in my opinion, it is the future. On the other hand I don't think it will make "old" Spring applications to be rewritten "just-like-that". Spring will be the most favorited solution for a long time.
I think that making a certificate path which bases on Spring solution is a good idea, because there still will be many companies which will continue to use it, and they can become your potential employer :-)
Cheers!