This is an awesome tip that makes life much easier for those who write in languages other than English.
Another accented character that is commonly used in Portuguese (Brazil) is "OPTION+I" that gives "", and typing "o" will write . Those writing in Norwegian, Swedish and Danish will love special characters like "OPTION+A" = "" or "OPTION+Q" = "" and German speakers "OPTION+S" = "". You can find out more by experimenting with the Option key. Enjoy...

Great! So now I don't need to have the Spanish and German keyboard layouts rarin to go up on the menubar. Sure, it's nice to have the American flag up there, makin you feel patriotic effortlessly, when Spanish or German isn't needed but using option and the letter will be so much better a solution. (I haven't tried the umlaut but hope it is there somewhere and that I can remember it. I assume this will show up on the US keyboard layout when option is pressed although that may mean I have to have the keyboard layouts up on the menubar again...)

lipwak, pressing Option "U", then appropriate letter will give you the umlaut marking.

This is a great tip for accented characters.

One thing that I really miss from PC and can't find on the apple keyboard is the superscript 2 and 3...

Any idea how to quickly get them ?

Well, I use the US International on my MBP and its great.