[13ren]

I think you're right, that it's simply US vs. British spelling, but I've always felt that the usage is slightly different (and that therefore maybe they are actually different words...).

An asshole seems to be someone you are angry with (legitimately or not). It's about behaviour (or perception thereof); but to call someone an arsehole is a darker, earthier insult, and is closer to what they intrinsically are. It seems more literal to me.

Maybe this is because I've mostly heard "asshole" on sanitized TV; but I've heard "arsehole" in uncensored RL.

PS: Firefox then spellchecked in a way that was almost but not quite entirely unlike British English.

[michaelneale]

That sounds reasonable to me, it does seem darker in the British usage.

Also - in US English, you could call someone an "ass" - its not really that bad an insult, like you said, more about their behaviour at a point in time. There is also the donkey meaning (ass) which makes it less insulting then it would otherwise be.

[13ren]

Yes - the donkey meaning. I think actual donkeys might also have been more common in the USA.