I live in Australia. It took about 2 days to get totally comfortable driving on the opposite side when I visited the US. I don't think it'll matter that much.
That said, my brain takes the cue from looking at what side the wheel is on (I just keep the side with myself on it close to the centre line). If I got off the plane and into a McLaren F1 then I'd probably have real problems (thankfully Avis doesn't carry twenty-year-old supercars).
Now I'm driving my Left-hand-drive Jeep on the left side of the road in Southern Africa.
I have also driven right-hand-drive vehicles on the left in Canada.
It's annoying for merging and overtaking, but it's not "difficult".