Another example could be found in Digium's Asterisk project : https://issues.asterisk.org/jira/secure/DigiumLicense.jspa
For more information on this topic of accepting contributions on dual licensed projects, I found the reading of this article of the CivicCommons wiki worth a read: http://wiki.civiccommons.org/Contributor_Agreements/ . In particular, it provides a detailed explanation of the difference between Contributor License Agreement (CLA) and Copyright Assignment Agreement (CAA).
Which one should you use?
The short answer is: CLA.
For government projects, we recommend CLA rather than CAA, because a CLA is physically easier for contributors (it can be submitted by email or web form) and because some contributors may sign more readily when no actual transfer of ownership is involved.
CAAs, on the other hand, are mainly requested by private-sector corporations who seek to consolidate copyright ownership. There are various reasons why a corporation might want that. The most common are that it can simplify assertion of copyright internationally, and it can enable “dual-licensing” models where the corporation makes the software available under proprietary terms, for a fee, to customers who need exclusivity for one reason or another (such as marketing a proprietary product derived from the code base). But the transfer of ownership involved in a CAA, along with the possibility of dual-licensing, makes it harder to persuade some contributors to sign, even when the CAA contains a “license-back” provision ensuring that the contributor retains all meaningful rights with respect to their own changes, or even when it contains a promise to only distribute the assigned works under open source terms.