One thing that I always found a bit odd, (mainly as the descendant of a group of Polish living along the Baltic), is that in CK1 Poland was always represented as a Kingdom no matter what.
Infact during the Middle Ages Poland had a King for only certain periods, most of the time Poland was infact an independent Duchy, ruled by the "Duke of Poland", and rulers would only assume the title of King occasionally through out history until it became much more common later on. In fact there was no consistant "King" of Poland until 1295. Earlier Dukes of Poland assumed the title if they gained in game terms enough prestige but 60% of rulers of Poland during the game's timeframe were called Duke. What I'm basically saying is that there should be two titles, Duke of Poland as well as King of Poland.
In 1066 in the games start date, Bolesław II was referred to as Duke of Poland. For a brief period of time he referred to himself as King from 1076-1079 but after his death all rulers of Poland instead had the title of "Duke" (or High Duke) until the beginning of the 14th century. Basically I think to be historical all you have to do is implement a new title "Duke of Poland" (with King of Poland obviously creatable) and have Poland ruled by a Duke. For referencing see here http://en.wikipedia.org/wiki/List_of...rchs_of_Poland.