Windows 7 taskbar progress works in one app but not another
My problem was likely specific to Delphi/C++Builder and was the result of a difference in how the VCL handles its top-level window.
If Application.MainFormOnTaskBar is true, then ITaskbarList3 only works if Application.MainFormHandle is used as the HWND parameter.
If Application.MainFormOnTaskBar is false, then ITaskbarList3 only works if Application.Handle is used as the HWND parameter.
In my case, one project had MainFormOnTaskBar true, and one had it false.
I have not written an app using the Windows 7 progress bar specifically but I have written many .Net apps that require updating progress using a live animation and the biggest gotcha from my experience was being sure you were updating the progress bar on the main thread.