I can't claim the credit - in England there are lots of superstitions about magpies, some people salute them (to reduce their power as they represent the devil), others say the poem above (or other similar poems) and give themselves good or bad luck according to how many they see.
Oh, I thought this was just a kind of rhyme, didnt know it was meant for magpies only. Thank you, now I have learnt something new again :)
As far as I know in Sweden there are no superstitions about magpies instead we have an old saying: spit three times if a black cat cross the road in front of you!