Is first come, first served hyphenated (first-come, first-served? Thank you.
Depends on how it's used in a sentence:
This is set on a first-come, first-served basis. (Hyphens necessary because it's defining "basis")
Reservations are first come, first served. (No hyphens necessary because it appears after the verb "are")
Edit: I am citing the Chicago Manual of Style "Compounds and Hyphenation" Sections 7.82 and 7.90 on "Compounds According to Type." For phrases such as first come, first served, I would only hyphenate if used as an adjectival phrase as I've indicated.