This is a question thats come up many times. To put some perspective into the argument have a read at this article:
http://www.bitcoinnotbombs.com/bitcoin-vs-the-nsas-quantum-computer/Please pay particular attention to the section quoting Bruce Schneiers 1996 book, 'Applied Cryptography' and the effects of thermodynamics on the processing of information, coupled with Peter Shor's algoritm for a quantum computer to break public key encryption. It's east to forget about physical laws that don't at first glance seem to have any influence.
The basic message is : SHA256 is safe, period. ECSDA
might be vulnerable
if a viable quantum computer can be built and run reliably but that at the moment would seem to be some years away ay present!
On a side note I've just finished reading an excellent book by Simon Singh, 'The Code Book'. It was written some time ago, long before Bitcoin came into being but it's a very interesting history of codes and codebreaking. Also explains RSA and why super large prme numbers are so sought after.