The eRx incentive is the same as 2011 and pays 1% of Medicare annual encounter charges. To qualify, report at least one eRx during 25 or more unique patient visits during the reporting year, by submitting the G-code (G8553) when performing the patient visit. More details
here.
Now the stick: If you have not submitted 25 successful eRxs by the end of 2011 or during the first 6-months of 2012, Medicare will impose a 1.5% payment reduction for charges submitted in 2013.
You are not eligible for the eRx incentive if you attest for the EMR incentive in the same year. However, you are
still liable for the penalties if you fail to perform at least 25 unique eRxs at a patient encounter visit. (How you would qualify for the EMR incentive and not meet criteria for eRx seems impossible to me...but then again, it's Medicare.)