Building v8 with mingw32 is not quite stable yet.
In the latest revision of the trunk, you need to tweak some piece of code in platform-win32.cc for it to compile.
Then, when you do get it to compile there is a nasty bug using a get accesor inside a anonymous method, wich doesn’t crashes on other platforms/compiler.
v8-users group discussions related to this bug.
In conclusion, to avoid losing your time like I did, you should use MSVC 😉