Until now i have always used the 'two space' rule after a period.  The only reason i can put my 'two space passion' down to was seeing my mother typing when i was a kid and asking her why she put two spaces after the period.  It was, as Bill points out in this video, was because in a fixed width font system you needed more space after the period to make it readable.  Funny thing is ... that i have just realised that as i am typing this i am still putting two spaces in :)  It is a natural thing for me now... but something i will change after watching this video.  Bill is funny to listen to.

http://channel9.msdn.com/ShowPost.aspx?PostID=112