As a little trivia note, the original script (or, at least, the fourth draft) of 'Terminator' had the line 'I'll come back' instead. It's also worth noting that 'I'll be back', as a catchphrase, first appeared in the 1980 vigilante exploitation semi-classic 'The Exterminator.'

'I'll come back' lacks a certain mythic resonance. The other famous lines from that film ('Fuck you, asshole' or 'Uzi nine-milimeeder') would probably not have caught on in the same way.

'I'll be back' is also a neat way of making it clear to the audience that, although Arnold Schwarzenegger is playing a different character in each of his films, he is still 'Arnold Schwarzenegger', or at least 'The Arnold Schwarzenegger Screen Persona'. James Cagney, Henry Fonda, John Wayne et al all played variations on the same character in the majority of their films; Schwarzenegger wasn't even playing a chracter, he was 'Arnold Schwarzenegger'.