Mental Note

Writing myself a mental note in case this happens again. I guess it's not really a mental note if I write it down though is it...?

When moving a user control that referenced another assembly into a user control within that assembly, remove the assemblyname attribute from the clr-namespance declaration. It will spit the dummy and tell you that the tag does not exist in the namespace when you can clearly see that it does. Another idicator that this is happening is if you hover over the InitializeComponent() method in the constructor it will ask if you want to generate the method stub.
So if you've got :-
xmlns:poo="clr-namespace:CompanyName.Product.Client.Support.Controls.Reports;assemblyname=CompanyName.Product.Support">

and you're now using the poo reference from within itself it will not compile. You need to change it to :-
xmlns:poo="clr-namespace:CompanyName.Product.Client.Support.Controls.Reports">

While I'm on it, another issue that I came across yesterday was getting a dependency property to work from within a usercontrol. I had it all wired up correctly but it didn't seem to be working. Upon consultation with the guru it was due to the fact that I hadn't specified the DataContext of the usercontrol to itself.

Referencing an image from another project in xaml

I recently had an issue where I had a usercontrol that was packaged in a project together with an image that it referenced. Another project used the usercontrol and it wasn't displaying the image. After fiddling with the image source, changing it to a pack URI and a few different things I found that it had nothing to do with the source property. The answer was to go to the properties of the image and change the Build Action to Resource instead of Content.
There you go.