UPDATE - Newsgator have now fixed this with 2.5.11.xx
I use NewsGator for all my RSS reading needs, but recently (when I was wondering why I seemed to have extra time on my hands) I remembered that I'd had to disable it while I was in Adelaide talking about MapPoint Web Services to the ADNUG and about VSTO 2005 to the mad-keen devs at SDM. I had to disable it because the current version of NewsGator is a FW 1.1 app that doesn't support FW 2.0. In that article they suggest that to make NewsGator work on a machine with 2.0 of the framework installed, you should create ot update the Outlook.exe.config file so Outlook only loads v1.0 or 1.1 of the framework.
One of the great new VSTO 2005 feature I was showing off was the new support for Outlook Add-ins. This is a great step forward - not least because it moves VSTO from a document-only technology (as is the case with the Word and Excel functionality) to covering application-centric add-ins as well. However, If you try to load a VSTO Outlook add-in into Outlook when you've constrained it to using v1.0 or 1.1 only, obviously things don't work. When you try to load the add-in you get a message saying that the load failed (but not why). This is because Outlook loads the framework in-process and can only load one version of the framework in-process. The only way to make this work is to disable NewsGator and un-constrain Outlook (either remove the constraints from the outlook.exe.config file, add another tag to the config file to allow FW2.0 to be loaded or delete (or rename) outlook.exe.config if there's nothing else in it).
Update - the text of the error message that appears in Outlook when trying to load the add-in is:
The customization assembly could not be found or could not be loaded.
You can still edit and save the document. Contact your administrator or the author of this
document for further assistance.
Update II - this topic is also covered on the MS Technical Forums, and according to one poster there, NewsGator will be releasing a 2.0 version soon.