UmwamleLaba - I like the general idea of your post, but I do not agree with the sentence "Poland has been a unified country for a very short period of time (maybe 200 years total)" - I was learning history of Poland for many, many years (as everyone in Poland) and AFAIK Poland is recognized as a country from year 966 (Baptism of Poland). The only sure period that we were not unified country was 1795-1918, so there is still ca 900 years left... We may argue about period around XII century, when there was several Princes of Poland, but as I remember there was always one senior Prince that hold the title of King of Poland.
FoxTrot12 - maybe funny, but offending.
Vertico - I'm not sure if vodka was invented in Poland, many countries like to claim this "invention"

surely we make one of the finest vodka now (we call it "wódka" read: "voodka", it comes from word "woda" which means "water" in Polish).
Ravo - I would gladly see Lithuania in civ, but to be honest I don't think that before Poland

Lithuania controlled larger area - it's true, but 1) those areas were hardly populated those days 2) Lithuanians made up very small portion of the controlled population and 3) in Commonwealth the main power was Poland.
Eskel - I would like to see Polish-Lithuanian Commonwealth

but i agree that it would be too hard to understand for foreigners - this union was in effect so long time ago that except for Poland and Lithuania people don't know anything about it. I think that it would be 'mission impossible' to convince Firaxis to add such civilization (unless next add-on would be about medieval Europe, then it could be great idea).
a4phantom - Slavian civilization would be strange for me. Mainly because Slavians make up the largest ethnic group in Europe and they would be represented as one civilization, while western civilizations are split into countries... strange. Secondly, Lithuanians does not qualify to this group - they're different ethnic group. Finally, Russians are Slavians as well - so it would be strange to have both civilizations.
I think that adding Poland would balance the representation of Europe in game. I know, it's already represented quite well if we take into consideration count of civs, but Europe is the craddle of modern civilization. It would have also positive impact on sales for Firaxis, because:
1. Poland is an emerging market - we have almost 40mln of citizens with GDP growing almost 6% a year
2. We have up to 20mln people with Polish roots abroad - with up to 10mln in USA alone
3. Slavians in Europe would be more than happy to have two Slavic civilizations in game. Russians are generally not liked in East and Central Europe as they occupied all of it during ca. 1945-1990 and imposed puppet regimes
I don't want to say that adding of Polish civilization will boost civ5 sales, but I think that it will undoubtely increase it.