Don't expect to open a line, grab the $100 credit, and desert the line. Nope, AT&T is one step ahead of you there. To qualify for the credit, the newly activated line must remain active and in good standing for 45 days. And AT&T will give you the credit some time within the first three billing cycles.
If you were planning to add a line to your existing AT&T account, or were planning on switching to the mobile operator, might as well do it before March 31st, because that is when this offer expires.
source: AT&T via AndroidCentral