There's no contest. Mashup has no hyphen. Although a Google search turns up more results for mash-up than mashup (2.53m to 2.25m), wikipedia defines "mash-up" as a music remix (the original meaning of the term) but gives "mashup" as the sole spelling for a Web 2.0 remix.
Back to Google and run a search for "mashup" "web 2.0" and compare the resultsMake that 301,001 to 68,699 to those for "mash-up" "web 2.0" and there’s no contest: 301,000 compared to just 68,700 (I am based in the UK so Google defaults me to its UK site whether I like it or not — very Microsoft-like behavior if you ask me — but I assume the results are the same using the US site or indeed any other country site).
The question came from ZDNet blog editor David Grober in response to the title of my previous posting, which inserted a hyphen even though David Berlind is deep in the throes of organizing a (hyphenless) Mashup Camp over on BTL. Obviously a respectable title like ZDNet has to maintain editorial consistency, down to the last hyphen.
Having checked the numbers I concede defeat and have edited the title of my earlier entry accordingly. Make that 301,001 to 68,699. I'm a stickler for respecting common usage and it stands to reason the text-messaging generation is going to drop the hyphen at the first opportunity, even if the mainstream media takes longer to follow suit.
For those of you interested in the linguistics of this (forgive me, I graduated in English at UCL, which is a world leader in the study of English usage, although I probably forgot most of it by now), the problem stems from the propensity of English speakers to create new words by adding prepositions to existing words. The origin of mashup is the phrasal verb "to mash up" (which you'll note has a space separating the two words, not a hyphen). Used as an adjective, the correct practice would be to hyphenate the two words (eg, a mash-up application), but the hyphen tends to disappear when the word becomes a noun (eg, an enterprise mashup) — compare "start-up company" and "startup".
PS: While we're talking about hyphens, a quick note about "on demand". Originally, this is an adverbial phrase which, strictly speaking, should be hyphenated when used as an adjective (eg, on-demand applications) but not when used adverbally (eg, applications delivered on demand).
So I hope that's cleared things up for everyone now.