Only about 10 printers are supported by apple to work properly with the latest iOS for the iPad which is 4.2.1 - have you installed it?
Pulled this list of a google search
The 10 printers that currently work are as follows. The HP ENVY100 e-All-in-One, HP Photosmart Premium e-All-in-One and HP Photosmart e-All-in-One, the LaserJet Pro CP1525, HP Photosmart Premium Fax e-All-in-One, HP Photosmart eStation, HP Photosmart Plus e-All-in-One, the HP LaserJet Pro M1536dnf and finally the HP LaserJet Pro CM1415f.