Das ist in etwa der Sinn. In den 70er Jahren gab es z.B. in dem Buch: Zen and the Art of Motorcycle Maintenance einen Passage zum Thema The Dialectics of Hip and Square.
d.h. hip ist das Gegenteil von square (spießig) und woimmer man hier dabei sein soll, das ist hip
(deutsch "in" ;-)).
Wann diese beiden Begriffe zuerst auftauchten weiß ich nicht, womöglich in den 60iger Jahren
Google-Suche nach "hip and square" gibt viele Beispiele

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Note added at 2003-06-07 03:56:26 (GMT)
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This is what I found about the origin, but it is just quoted from the internet:
Be there or be square!

An expression suggesting that all the cool kids will be a certain dance or party.

This expression was popularized by Dick \"Hug\" Huggyboy in Los Angeles, advertising dances at the El Monte Legion Stadium. It eventually spread throughout the country as a slangy catch phrase.
Response from Susumu Enomoto (Shiraokamachi - Japan)
Also: Be there or be square came from my advertising in los Angelas radio show. I was in Las Vegas the producers of the show. Saturday night dancers at the El Monte legion stadium called me on the phone on a Monday night his name was Al Zieger. Who was also the producer of the borsh capades. Starring \"Mickey Catz\" and said what are you doing in Las Vegas? We put together the acts for Saturday nights at the american legion stadium and the radio station needs the advertising spots for saturday nights. I said Al i\'ts only Monday night Itold him how can I get there so fast. Why don\'t you get someone else to read the spot,for the rock and roll show? Only you could do the spot! because our customers love to hear you say Be there or be square! so get your ass on a airplane and be here in Hollywood by 7:00p.m. or your fired! (same site)
and: In 14th century England, laws proclaimed the permitted length of the pointed toe (your shoe or boot), depending upon social status. Heels, after their introduction, soon indicated the status of the wearer by their height. In fact, the term \"well-heeled\" originated fro the wealthy wearing high-heeled footwear. Originally, if you were called a \"square\", it referred to the fact that you wore square-toed shoes well after they were out of fashion.
http://www.wordwizard.com/clubhouse/founddiscuss.asp?Num=292...

More from this site: http://216.239.39.100/search?q=cache:76u6lbC0LmkJ:home.attbi...
Starting in 1958, and through the early 60’s, I worked in broadcast radio in the era of real “Rock and Roll”. I was a DJ at several LA stations, KPOP and KDAY and also worked with the rock n’ roll shows in and around the Los Angeles area. If any of you were in the LA area you’ll remember the radio blaring things like, “El Monte Legion Stadium, this Saturday Night” or the immortal phrase of “Be there or be square”. Other places that you heard mentioned were the Harmony Park Ballroom in Anaheim, or the Long Beach Municipal Auditorium, with the ol’\' Red Cat, Art Laboe, or Huggie Boy.

das darf man nicht verpassen,

but, with b) I can't help, just heard somewhere as well.

Würde ich spontan sagen.

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Note added at 2003-06-07 07:53:00 (GMT)
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Oder auch: Mach mit, sei kein Spielverderber

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Note added at 2003-06-07 08:18:30 (GMT)
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Oder: Mach mit und sei kein Spielverderber

square = spießig, Spießer

Wer nicht kommt, der will nicht kommen, weil er als Spießer nichts mit der Veranstaltung/Party etc anfangen kann. Das ist auch ganz gut so, denn Spießer sind eigentlich unerwünscht und sollen daher wegbleiben. So deute ich den Spruch.



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Note added at 2003-06-07 10:31:13 (GMT)
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And again in English:
This is an invitation to an event/party of major importance, and is related to the expression ‘‘everybody who is anybody will be there’. The implication is that if you don’t come, it’s because you’re ‘square’, not part of the in-crowd, a ‘nobody’, somebody who doesn’t realise what a wild/happening/cool etc event this is going to be. In which case, let’s face it, you wouldn’t be missed.