Well, from Heir of the Empire on anyway.
Yoda said that Qui-Gon Jinn found a path to imortality, so basically, anyone who learned this from him (Yoda, Obi-Wan and in a round about way Anakin) would be around forever.
In Heir to the Empire, Obi-Wan appears to Luke Skywalker in a dream and says that the bonds between him and Luke was going away and he would not be able to communicate with him anymore thus leaving Luke on his own to restore the Jedi order to the galaxy.
If we go by what ROTS puts forth, Yoda, Obi-Wan, Anakin and even Qui-gon could be around to advise Luke and the other Jedi in all the problems of what the Jedi should do, act and whatnot throughout every EU story post ROTJ!
Now I'm not going on some crazy book burning crusade or anything, personally there are many EU stories I really enjoy, but this little tidbit's been kind of bugging me lately. Granted none of the authors knew that this was how GL was going to establish how there could be Jedi spirits running around in the OT, but they could've made it possible to communicate with them, and possibly even train the technique to future Jedi.
Think of how different the NJO series would've been if we could've seen Anakin Solo again?
Oh well, I just thought I'd share my thoughts on this and hear what the rest of you would have to say on the subject.