Most of our users started out with revision 1 of the Raspberry Pi, but a growing number have revision 2 boards, and some have both.
A few things changed in revision 2, including GPIO pin numbering and the I2C interface.
Until yesterday our GPIO library did not handle header pin 13 correctly on revision 2 RasPis, and the I2C library required you to specify that you wanted to use I2C bus 1.
We've updated the library and those problems should now be a thing of the past.
If you are using the quick2wire-python-api libraries, you will need to pull the new versions from github. If you got to the directory in which you cloned the library, just run git pull from the command line and you should get the new version of the code.