Nice part of the world to come from Hairychested.
The latest issue of Singletrack has an article about riding in the Bieszczady mountains in Ukraine. This range also continues into south-east Poland so this could be another option for you. However i am told there are still wolves and bears living there. I am sure Hairychested could advise you further on this.
I have also been walking in the High Tatras near Zakopane on rocky trails that would have been awesome to ride but, sadly, i think they were for hikers only. The locals were riding them on quad bikes but I think this was more to do with it being part of their job rather than any recreational activities 😉