Why didnt Celeborn go with......

[Voralphion]

Maybe he just wasn't invited to go

All of the eldar were invited to go to Valinor. Celeborn was certainly able to go, he just wasn't finished with middle earth yet. Galadriel having already been in valinor grew weary of middle earth quicker than him, and only because the ring preserved some of the eldar days was she able to bear it, forgetting the fact that she wasn't allowed back yet. After the rings lost their power, she didn't want to remain in middle earth.

I think it's really unfair that, just because Aragorn was king of Gondor, he was the only remaining (ie, not Boromir) member of the fellowship who wasn't allowed to come to Valinor.

Pippen and Merry alos didn't go to Valinor. The only reason that Frodo and Sam were able to go west with the elves, was because they had both bourn a ring of power. This was such a burden and it altered their existence slightly so that the were given a reward and a chance of healing in the undying lands. The reason Gimli was allowed to go was because of the great friendship between him and Legolas and because of his great love for the lady Galadriel. The opportunity to go to Valinor was not given to the whole fellowship, but exeptions were given to three of the members who wouldn't normally have been allowed to go.