I think it's a bug, so I asked a question in this group (django-developers).
Ok - *that* is a different matter. For future reference, if you're suggesting that something might be a bug, it's helpful to make that clear in your question.
As I said in my response on django-users, the file based cache is primarily there as a proof of concept that the caching API can be targeted at multiple backends. However, if someone were to propose a patch to add file locking or temp file write-throughs, I don't imagine it would be turned down.
So - feel free to open a ticket for this; and if you want to try your hand at a patch, this should be a relatively simple patch to create (although tests could be a little bit complex). As a helper, the django.core.files.locks module already contains some OS-abstracted code for file locking.