The Creative Commons FAQ says "Creative Commons licenses ... are all non-exclusive".
Section 8e says the following:
- "This License constitutes the entire agreement between the parties with respect to the Work licensed here. There are no understandings, agreements or representations with respect to the Work not specified here. Licensor shall not be bound by any additional provisions that may appear in any communication from You. This License may not be modified without the mutual written agreement of the Licensor and You."
For clarification, see the mailing lists or forums.