Beginning June 5th this year, Era (www.era.pl) will turn into T-Mobile (http://t-mobile.pl). PTC (the network's operator) is owned in 97% by Deutsche Telekom and it is now finally turning into the global mark. The current prepaid (Tak Tak) and PTC's MVNO (Heyah) will remain in tact. Couple of interesting things from T-Mobile's PL FAQ:

T-Mobile PL will start brand new plans on the day of rebranding (June 5th). Example sinclude unlimited talk within T-Mobile network or completely new ROAMING offers. Roaming prices will be much much cheaper - they will actually be domestic prices abroad

That last bit is very very interesting. It is unclear whether it will only be within other T-Mobile's (like only T-Mobile UK etc) but we will see in 2 weeks.