From the release notes for the development version of django 1.2 I found this.
One bug we have found through using version 1.0 and 1.1 is that django also fails to understand cookies with a ":" in their name. This makes the login system completely fall over, so it mite be worth encoding those too. We discovered this bug when running django under the same domain as another tool, and it was settings cookies such as blah:wiki, and once those cookies were set upon that domain, we simply could not log back into the django section of the site without first clearing the offending cookies.
Hopefully I have provided enough information to help you fix this bug.