I'm using Phonegap 2.3 and I'm considering updating to the newer library. I think this might be a problem that was fixed with the newer library.
Anyway I have tried:
window.open('http://google.com','_blank')
window.location = "http://google.com";
Both had no luck forcing the user to load the external link in safari.
I also ready about targeting _system
which didn't work either.