- if you use framebufferObjects, you shall add glBindFramebuffer before glReadBuffer and glReadPixels but I don't thing the problem is there
- have you checked glReadPixels doesn't sent an array of 0 ?
- I use the lib Devil (new OpenIl), with this code :

ilInit();
iluInit();
...
ILuint ratz;
ilGenImages(1, &ratz);
ilBindImage(ratz);
vector<GLubyte> store;
store.resize(Wide * Height * 4);
glReadPixels(0, 0, Wide, Height, GL_RGBA, GL_UNSIGNED_BYTE, &store[0])
+;
ilTexImage(Wide, Height, 1, 4, IL_RGBA, IL_UNSIGNED_BYTE, &store[0]);
ilEnable(IL_FILE_OVERWRITE);
ilSaveImage(sortie.c_str());
[download]

where sortie is a string containing the file to write

Hope it should help