[icegreentea]

Well, they're speaking in rigourous terms. Quantum computing will allow us to attack an enlarged set of problems in polynomial time. For example, integer factorization. However, there are still going to be other problems that quantum computers cannot solve in polynomial time (ie, much better than classical computers). For example, it's generally believed that they will not be able to solve NP-Complete (traveling salesman) problems in polynomial time.

For a nice description and diagram, take a look at the wiki page. http://en.wikipedia.org/wiki/Quantum_computing#Relation_to_c...

[swordswinger12]

Shameless piggybacking: Also look into the work that complexity theorists have done with quantum complexity classes like BQP.