Heya,
Yeah, feeling a little ashamed of my conviction to be honest: I have been such a hard-ass opponent of all IP elsewhere on the web that this one is close to going completely against my principles... Lets just say I am such a Vonnegut worshipper that this stuck in my usually Anarchist/Libertarian craw.
But, being a hardass opponent of IP does mean I have a passing knowledge of the appropriate laws, so:
The work was first published in 1961. Under US IP law previous to the Copyright Act of '76 there was a requirement for renewal after twenty eight years after publication, whereas the '76 law turned it to life + 50 years and the Sonny Bono act made it life + 70 years... Therefore unless Vonnegut has since handed this over to the public domain (I can't find a record of this) then it's still covered as copyright, and will be covered up till 2077 (stupid bloody Sonny Bono, it's obscene)...
I think it is the brevity of this piece is probably the reason it's across the web, it is commonly taught in schools and I suspect that some obliging lecturer has stuck it up as class-notes. I dont know what the legal status is of the academic use of copyrighted material in English teaching, but I suspect it's probably a generally accepted minor-league copyright violation. Further, I don't think it counts as fair use (since the work is printed entire without addition or adaptation)...
I don't know whether Vonnegut himself handed this over to the PD, or Creative Commons... But in a 10 minute search I couldn't find conformation that he had. Wouldn't be in the least bit out of character tho, and it's probably necessary if your work is going to be done at length in schools.
On the other hand there are lyrics plastered over the entire web, which shows there is a general usage consensus that sufficiently brief pieces can be used in any form you like... Or to put it another way: Sorry poets, but you guys are out of business.