Should adjectives that refer to people or cities be capitalized? "Live a Spartan life" "Life is Roman in its opulence."?

Proper nouns get a capital.

And if you turn these proper nouns into adjectives they remain capitalized.

If a proper noun enters the language, it is treated as an ordinary word and not capitalized:

Andrew’s college dormitory offered truly spartan accommodations.

When it actually refers to the person or place named, it retains its capital:

The Spartan army was notable for its rigorous discipline.

Note that Sparta was a Greek city-state and did not have anything to do with the Ides of March.

“Roman” has not become an English word with a meaning of its own separate from a reference to Rome, except in the case of Roman numerals (still usually capitalized) and roman typefaces (usually lowercased), meaning regular upright characters rather than italicized. So it should still be capped.

If you are using “spartan life” meaning frugal, then no, you don’t capitalize. If you are using “Spartan life” to mean a life that has characterisms of someone from Sparta, then you do capitalize.
As for Roman, the closest thing in English would be roamin’. Roman, of course, has to remain with the capital R. (Besides, the Romans didn’t use lower case letters anyway.)

@pothead: when a proper noun becomes an adjective in its own right (like spartan or lesbian) it is not capitalized. can anyone think of of any other proper nouns that became adjectives?

and in answer to your question, i would say don’t capitalize, maybe italicize for emphasis?

A good joke or play on words doesn’t need added emphasis.

A word that is derived from a person’s name is called an eponym. The language is full of them; some retain the cap and some don’t. A currently popular one is “maverick.” Here are some lists:
http://users.tinyonline.co.uk/gswithenbank/eponyms.htm
http://en.wikipedia.org/wiki/List_of_eponyms
The Oxford University Press has a whole dictionary of them:
http://www.encyclopedia.com/A+New+Dictionary+of+Eponyms/publications.aspx
Lists of place names that have evolved into words would probably be nearly as long.

Thanks to everyone for your help! I think I’ve got it.

Wait up! When was “maverick” a proper noun except as the title of a TV show? It’s a stray calf. The Maverick brothers (and cousin Bo) were like studly, unbranded strays, hence the show’s whimsy of a title.

(I think I’m just venting my sick-and-tiredness of the recent overuse of “maverick.” Let it die. Hope it comes back later as its former self.)

Didn’t say it should be capitalized. Said it was derived from someone’s name.

During the preelection period, the Maverick family of Texas, from whose name the term is derived, objected to the way it was being used in the campaign.