[ajarmst]

IANAL, but my understanding is that if their contribution is a derivative work (as most would clearly be), they would be subject to the same license restrictions that restricted their access to the original project. I.e. they had no permission to create the derivative work, and thus no right to release it under any license. If the contribution isn't a derivative work, then the project shouldn't be accepting it without a copyright assignment or license.

[OJFord]

That would make sense, but then what are 'Contributor Licence Agreement's for?

[tpxl]

If you contribute to a GPL project, your contribution is licensed under GPL, but it's your copyright, not the project owners. The Contributor License Agreement transfers the copyright from you to the project owner, so they may re-license (or dual license) in the future.