You could use http://www.inside-r.org/packages/cran/dplR/docs/uuid.gen from the dplR package.
gen <- uuid.gen()
uuid <- gen()
What's nice is you can easily alter the state of the generator with the more_state argument of uuid.gen.
Thanks to Carl Witthoft for pointing out that my first version was totally broken. Turns out calling sample with replace=TRUE greatly expands the possible UUIDs you might generate! :)
Carl also says, "In general, as I understand it, the value of UUID codes is directly dependent on the quality of the pseudo-random number generator behind them, so I’d recommend reading some R-related literature to make sure “sample” will be good enough for your purposes."
This sounds wise, but I'm not sure if I'm smart enough to follow up on it. It could be that the randomness of these UUIDs is less than ideal.