There's no catch.
Banks need to acquire customers just like any other business. One common way to acquire new customers is by advertising on the radio, TV, print, etc. Another common way to acquire new customers is by offering incentives like the one you linked to.
Basically, PNC is confident that they will make more than $100 in profit over the entire lifetime of a customer. This is a very reasonable assumption, considering that:
- In the second quarter of 2010, PNC Financial reported $851M in net income.
- According to Wikipedia, they have approximately 5M customers.
- Therefore, net income per customer was $172 for just April through June 2010.