I downloaded the official prebuild package for MinGW32 from qt-project.org, and tried to build next source:

int main() {
	return 0;
}

TEMPLATE = app
TARGET = bug
SOURCES += main.cpp

When I ran qmake and mingw32-make in 'bug' folder, I got next error message:
g++: error: release/main.o: No such file or directory

I found releasemain.o file in 'bug' folder, and I'm guessing the Makefile generated by qmake has some issue about slash/backslash on windows plaform.
Actually, when I replace all backslash() in Makefile.Release with slash manually, mingw32-make works fine.
I think there's no need for using backslash even on windows, at lease for MinGW32 (I don't know anything about nmake with msvc) because mingw32-make handles slash well and backslash can cause some ambiguation problem with backslash for new line.