Consequences of Factoring being in P?
There are pretty much no complexity-theoretic consequences of Factoring being in P. This means that there are no good justifications for factoring being hard, other than that nobody has been able to crack it so far. 

Polynomial-time factoring would make it possible to take square roots over $Z_n$ (and also over a much more general class of rings as well), and give polynomial-time algorithms for a number of other number-theoretic problems for which the bottleneck in the algorithm is currently factoring.

As for practical consequences, banking transactions are probably not that much of a problem -- as soon as it was known that factoring was in P, the banks would switch to some other system, probably causing only a brief period of delays while this was being implemented. Decoding past banking transactions would probably not cause serious problems for the banks. A much more serious problem is that all the communication which was previously protected by RSA would now be in danger of being read.