Gregorz has an impressive website and details his mountain bike trips in the Beskids mountains on the southern Polish border (mostly bordering Slovakia, but touching the Czech Republic and the Ukraine as well) here:
http://www.santaclub.pl/teksty,relacje,2.html

His trips in other parts of Poland can be found here:
http://www.santaclub.pl/teksty,inne_pl,4.html

EnduroRiderz.pl is an impressive collection of content about Poland and its surroundings, including GPS tracks and maps

EpicMTB.pl does organized trips in Poland, and their site has a nice overview of trails in English.

And, who would have thought that Velo News has a full Polish site? It makes me wonder how many foreign sites they have.

http://portal.bikeworld.pl/

Kielce mountain biking portal

http://www.maparowerowa.pl/