Is there a way to enable / disable Add-In buttons / tools?
According to the documentation this can be done in the OnUpdate method, Add-in coding patterns
I got that wrong the first time . Not sure why I was thinking checked when you said enabled.
But enabled is boolean just like checked: http://help.arcgis.com/en/sdk/10.0/arcobjects_net/componenthelp/index.html#/Enabled_Property/001v000000s0000000/
For example, check out line #51 :[http://code.google.com/p/umbriel/source/browse/trunk/Umbriel.ArcMap.Addin/Umbriel.ArcMap.Addin.EditorTrack/EditorTrackToggleButton.cs#51]