One thing I find tricky is when I've known someone in Germany for some time, on quite a friendly basis, and then I start talking or writing to them in German - can I use 'du' rather than 'Sie', even though they've never formally invited me to do so? Usually I do - if I've made a blunder, can put it down to English-speaking ignorance! :-)
I recognise that situation as well, Ian

The convention can vary from region to region, and generation to generation. I knew an elderly married couple from a rather stern family background who called each other by the polite form after 30+ years of marriage, with three children. On the other hand in most of Siberia no-one uses the formal form at all, even to strangers - except, perhaps in shops, and not always then either. So the only rule is that there are no rules. (A Russian friend was rather taken aback at being addressed as "hinny" by shop-assistants in Newcastle...)
But terribly English/American to ignore those conventions entirely.
Perhaps not entirely? Although it gives some Brits the willies, the American "y'all" is a "polite plural" form, even if it may not quite sound remarkably polite

I did a whole show with a Texan tenor once, and he distinguished between the two forms very precisely. I've always imagined that it slipped into use by immigrants from countries which have the polite plural, who felt uncomfortable with the absence of one in contemporary English? They just translated what they would have said in their native language as near as they could. (A lot of American English expressions are like this - "in back of" (meaning behind) is translated word-for-word out of German, "im zuruck des".)
Like the German equivalent, the Russian polite form (Vy) is conventionally capitalised, whereas "ty" isn't. I have seen some English translations where they've capitalised the "formal version" to "You" in mid-sentence to show which one's been used in the original - this seemed to work quite well?
Opera fans are always given the example in Russian from EVGENY ONEGIN... when Lensky is wooing Ol'ga in Act One, he starts out in the first couplet with "ya lublyu
Vas, ya lublya Vas, Ol'ga...", but we hear him cranking-up the ardour by the time he gets around to the recapitulation - it's changed to"ya lublya
tebya". (In fact the entire opera is full of this - Onegin holds Tatyana at bay with a frosty "Vy", and by the time he's changed tack, it's too late... and a social faux-pas to address Countess Gremina as "ty").
http://www.youtube.com/watch?v=HCY6zNhN_cI