Heyah operator is the best choice for Travelers including Data Transfer and Calls.
Technically it works on ERA GSM network so it has same coverage as ERA. UMTS is avaible in cities, EDGE in almost rest of the counry and GPRS anywhere else. It has 100% coverage of country including Data Transfer and Calls. Coverage map is here.
Maximum data transfer is 1 Mbps on UMTS.

Of course it's a pre-paid offer and you can buy a SIM-CARD in almost any market or gas-station. You will recognize it by big red hand which is a logo of this brand. SIM-CARD costs 20 PLN with 20 PLN to use (1 PLN = 0.25 euro on august 2009).


Prices:

National calls (landline/cellular): 0.29 PLN
Calls to Heyah: 0.14 PLN
Calls to all EU Countries: 0.59 PLN - about 0.15 euro !!!
Internet: 0.02 PLN / 100 KB rounded every 100 KB - about 0.05 euro per 1 MB !!!! It's about 50 euro per 1 GB

If you're going to Poland with someone else you can also use
Technical info:

APN: heyah.pl
User name: heyah
Password: heyah

(You must use these settings, otherwise it won't work)