I've searched around a bit but didn't find a guide yet. Something with a checklist would be great.
- Potential backward compatibility issues in Joomla 2.5 and Joomla Platform 11.4
- Potential backward compatibility issues in Joomla 3 and Joomla_Platform 12.2
Nick also found this non-Joomla Doc's article:
There is a doc page explaining the potential backward compatibility issues between 2.5 and 3.x. http://docs.joomla.org/Potential_backward_compatibility_issues_in_Joomla_3_and_Joomla_Platform_12.2
Make sure you don't use any of the deprecated and removed methods found in there and the extension should work in J3.x.