As a speaker of the Queen's English, as distinct from the President's English (as in "Yo! Blair"), I am acutely aware of the differences between our languages. On my first trip to the US I mentioned that I was going to go and change into my 'swimming costume', and everyone fell about laughing. I admit that people are entitled to laugh when they actually see me in my Speedos, but I thought that it was a bit rude to laugh just at the thought. How was I to know that for Americans this expression conjoured up an image of someone emerging from a Victorian bathing machine, wearing a striped blue and white knee-length one-piece?
The topic (lingustic difference, not, thankfully, me in my beach gear) is important enough to have a very long (and to me rather dull) wikipedia entry, the highlight of which is the following:
In BrE, the phase "I can't be arsed [to do something]" is approximately equivalent to the American "I can't be bothered [to do something],". This can be extremely confusing to Americans, as the Southern British pronunciation of the former sounds the same as "I can't be asked...", which clearly sounds either defiantly rude or nonsensical.
Now it is true that I don't think that there is any documented example of the Queen using these words, but I bet her grandchildren do. Up to now it hasn't been a regular expression of mine either, but things are going to change, now that I have seen a potential explanation of its meaning. As I understand it, it is a shortening of the expression 'I can't be bothered to raise my arse out of this chair to do that.' In essence, then, it means 'I so cannot be bothered to do that that I can't even be bothered to utter a fully formed sentence to express the fact that I can't be bothered.' Brilliant. And very, very useful.