[baddox]

That analysis is probably confounded by the fact that "more fun" can be used with "fun" as a noun, as in "We had more fun after you arrived."

[sgustard]

Good point. A google search also shows that since "fun" has only recently migrated from a noun to an adjective, the -er/est forms are not yet accepted, but will likely become standard over time.

[samstave]

Funaholics.