Era -> T-Mobile rebranding has been suspened due to legal issues: Vivendi still fights with DT for 48% PTC shares.
Tomorrow (02.02.07 at 20:45 local time) P4 (which is "working title" only

) will reveal its brand. Commercial start should happen right after, though there are rumours about it may be only on 1st March.
At the moment, P4 has its 3G towers in Warsaw and Tricity (Gdańsk aglomeration on the north) and uses GSM roaming in Plus network elsewhere.
No rumors about rates yet.
Probably P4 and its landline shareholder Netia made takeover of Germanos Poland which has been an Era dealer network - this information is not confirmed however.