Since their previous France and UK accounts were closed, both Mt. Gox and Intersango set up accounts in Poland.
Is there a specific reason why they chose Poland?
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As for Mt. Gox, they got their polish account when they bought up bitomat.pl, after it lost its private keys. They continue to support accounts from that other website, so they allow easy money transfers to that account (although it takes them a couple days to credit the account).
As for Intersango, I presume they might want to cater to that part of the market (bitomat was dealing solely in PLN and it was the third largest Bitcoin exchange at a time). It could also be related to the fact that poles often work in UK (where the exchange is located) and transfer their money back to Poland - wire transfers between countries are quite expensive, while sending Bitcoins is cheap.