Believe it or not, someone else encountered the exact same problem AND found the solution!
The full solution is here, but in short, you need to run the following registry fix and the designer is back :)
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETCompactFramework\v3.5.0.0\PocketPC\AsmmetaBinder]
"TypeName"="Microsoft.CompactFramework.Build.PocketPC.BindingService, Microsoft.CompactFramework.Build.Tasks, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, Custom=null"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETCompactFramework\v3.5.0.0\PocketPC\AsmmetaBinder\4118C335-430C-497f-BE48-11C3316B135E]
"TypeName"="Microsoft.CompactFramework.Build.WM50PocketPC.BindingService, Microsoft.CompactFramework.Build.Tasks, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, Custom=null"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETCompactFramework\v3.5.0.0\Smartphone\AsmmetaBinder]
"TypeName"="Microsoft.CompactFramework.Build.SmartPhone.BindingService, Microsoft.CompactFramework.Build.Tasks, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, Custom=null"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETCompactFramework\v3.5.0.0\Smartphone\AsmmetaBinder\BD0CC567-F6FD-4ca3-99D2-063EFDFC0A39]
"TypeName"="Microsoft.CompactFramework.Build.WM50SmartPhone.BindingService, Microsoft.CompactFramework.Build.Tasks, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, Custom=null"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETCompactFramework\v3.5.0.0\WindowsCE\AsmmetaBinder]
"TypeName"="Microsoft.CompactFramework.Build.WindowsCE.BindingService, Microsoft.CompactFramework.Build.Tasks, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, Custom=null"