Hi, thank you for contacting us.
Current version 1.7.7 of To-do Lists application is compatible with Mac OS X starting from 10.6.6, so with Mountain Lion as well.
At the moment we are working on version 2.0 of the application, and currently deployment target is set to 10.8, so it should support Mountain Lion unless we'll found some major showstoppers.
Apple constantly changes APIs here and there, pushing hard developers to support only latest Mac OS X versions. However we are trying to not follow this rule and support as many OS X versions as it is possible. Basically, only now we are dropping support of 10.6-10.7 because it is a total pain to work with localisations and some other features.