First - the rendering from BIAB is one of the very best of it's features - real easy and always working for me. I can do all the options successfully on my iMac. Seven minute tunes have taken no longer than about 20 seconds to render.

Second, check your folder permission access (the folder where you sending the files). For some odd reason, when I installed Snow Leopard, access permissions changed on many folders and files. Set the folder permission to Read $ Write.

Third, I'd try using the mp3 render with the mix all option, first. Then try the AIFF...