"It indicates by a dotted underline which words it thinks you have misspelled."

I wonder if that will mean all the words where Americans use their own idiosyncratic spelling... Though the spellchecker built into Firefox 1.5 doesn't do that, it accepts both versions as correct. (Which means of course it doesn't pick up mistakes that happen to coincide with the spelling on the other side of the Atlantic. (But it thinks the word Firefox is a misspelling!) (Actually it's part of the Google bar - and it does accept Google as correct English.)