これ？
http://www.qtcentre.org/threads/4625-WCHAR-to-QString-giving-error-in-vs2005
http://stackoverflow.com/questions/5625884/conversion-of-stdwstring-to-qstring-throws-linker-error

> Edit your Visual Studio project settings and under C/C++ -> Language
> set the option Treat wchar_t was Built-in Type to No.