What is likely happening here is that you have a queryset created in 1.3 stored in the session. When Django tries to load and evaluate it, the stored queryset doesn't have the prefetch stuff in it, as 1.3 didn't have it. I am not an expert of the pickle protocol, so it might be this explanation is plain wrong...
This ticket is likely invalid. But, should we advice people to clear their sessions on Django version upgrades? If the above guess is correct, similar errors (which are hard to debug) are bound to happen...