T4 – <#@ Assembly name="$(SolutionDir)\T4Helper\bin\debug\T4Helper.dll" #>
As mentioned here and here the assembly referencing has changed from VS2008 to VS2010.
Say you have a T4 helper assembly aptly named T4Helper and you have it in a project called… T4Helper.
MySolution ....MyProject ........MyT4Files ........MyOtherFiles ....T4Helper ........T4Source
The VS-macro-way to reference it from MyProject is
<#@ Assembly name="$(SolutionDir)\T4Helper\bin\debug\T4Helper.dll" #>
It obviously has problems when switching away from debug compile. But the line above will help you get the backslashes right at least.
Update:
Something with
$(ProjectDir)$(OutDir)
might do the trick.