ArcGIS 10 Add-In compile problem - Why are new edits to code igonred by ArcMap?

I had a similar issue to this and resolved it by deleting the corresponding cache directory for my add-in:

%LOCALAPPDATA%\ESRI\Desktop10.<x>\AssemblyCache where x is 1,2,3,4, or 5.

C:\Documents and Settings\<username>\Local Settings\Application Data\ESRI\Desktop10.0\AssemblyCache\

10.1 on Windows7:
%LOCALAPPDATA%\ESRI\Desktop10.1\AssemblyCache will take you to:
C:\Users\<UserName>\AppData\Local\ESRI\Desktop10.1\AssemblyCache

I also have in my notes to check this directory, but I don't recall if that was for while I was debugging in Visual Studio or just normal troubleshooting. :
c:\Documents and Settings\<username>\My Documents\ArcGIS\AddIns\Desktop10.0
If the AddIn that you are debugging exists here, try deleting it here.

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Update

I knocked one of my add-ins and debugged. It does create a directory in My Documents\ArcGIS\AddIns\Desktop10.0 for debugging, which is unpacked to the AssemblyCache directory.