In his [Indiscrete Thoughts][1] Gian-Carlo Rota writes:

> Every mathematical theorem is
> eventually proved trivial. The
> mathematician's ideal of truth is
> triviality, and the community of
> mathematicians will not cease its
> beaver-like work on a newly discovered
> result until it has shown to
> everyone's satisfaction that all
> difficulties in the early proofs were
> spurious, and only an analytic
> triviality is to be found at the end
> of the road.

According to Rota what you ask for is the normal case. He - and others - do not even rule out the possibility that some day Fermat's Last Theorem turns out to be "trivial".


  [1]: http://books.google.com/books?id=nFvY20pHghAC&printsec=frontcover#v=onepage&q&f=false