GUIMiner won't start mining on Slush's pool
The problem is that in GUIMiner, the URL for Slush's pool is:


  api2.bitcoin.cz:8332


That address has been deprecated, and it falls back to the GetWork protocol. From Slush's Pool news:


  10.03.2013
  Default mining URL for Stratum is stratum.bitcoin.cz:3333. If you're still using api.bitcoin.cz, please fix your URL to prevent fallback to deprecated Getwork protocol.


In the GUIMiner interface, instead of choosing "Slush's Pool" from the dropdown menu, choose "other" and then enter

http://api.bitcoin.cz:8332

or

stratum.bitcoin.cz:3333