Mezzanine lead developer here.

We'd been tracking Django 1.5 changes during the release candidates and were mostly compatibile with it up until its release. When it was released, a few issues cropped up - those were resolved within a day or two, so there was a brief period where it was incompatible with 1.5, but that's no longer true.

So it was incompatible with 1.5 for less than 48 hours. Latest version works fine against 1.5, enjoy :-)