If I said “Tom’s going to be bummed”, depending on whether you were an American or a Brit, you would either worry about what Tom will be upset, or wonder why Tom was going to be… err… taken from behind.
Today’s NTK points out this difference. In the US, ‘bummed’ means ‘upset’ or ‘depressed’ – “he’s going to be bummed about his mum’s illness” would be perfectly reasonable. However, the verb ‘to bum’ in British English is, to put it bluntly, ‘to engage in anal sex’. To be bummed by someone would mean that you are on the receiving end of a, well, bumming. I suppose the word ‘bummer’ could also be used to describe someone who regularly likes putting his genitalia up other people’s rear ends, although I think any Brit that has had some exposure to American TV shows will understand it’s other meaning.
It’s just one of those occasions when you have to be very careful about what you really mean. I’d suggest reading the English to American dictionary – I know I link to this site a lot but it’s very interesting and humorous at the same time. And it may save you from embarrassing yourself.