Guava contributor here.
Guava doesn’t currently have an ImmutableEnumMap variant, but if it did, it would probably just be a wrapper around an EnumMap. (That said, slightly better immutable implementations are possible.)
EnumMap will perform better than the basic ImmutableMap, in any event; it’s difficult or impossible to beat.
(I’ll file an issue to investigate adding an ImmutableMap variant for enum key types, though.)
Update: Guava 14 adds Maps.immutableEnumMap().