就 and 便 are synonymous in many sentences:
I believe that 便 is more formal than 就; I've seen it in writing but never heard it in speech. However, my Chinese experience is mostly limited to the south, and perhaps it's common in other regions.
Is 便 a common subsitute for 就 in certain parts of China? In places where 便 is common, can it be used everywhere that 就 is used? Are there sentences where 就 is appropriate but 便 is not?