I've got a new test launcher for Thunderbird Portable 3 RC 2 posted. This is to fix the issue where Thunderbird 3 keeps asking to be default on Windows 7.
You can download the ThunderbirdPortable.exe launcher and replace the one in your ThunderbirdPortableTest directory to try it out. The source is here.
Please try it out and ensure it's working as expected on Windows 7 and other OSes. And ensure it works with 2.0 upgraded profiles as well.
Thanks!