Mav, you are right in that the effect is limited because we cannot retroactively apply the second license to past edits. But if we consider the effect, it seems it is still better to introduce it than not, and we would do just as Electicology suggested:
If "new" licence is chosen, then it could be noted that all contributions after a decided year would come under the new licence. If a contributor was also active before that date any contribution that he makes after that date will carry a retroactive application of the licence to his previous edits. We can deal with the others later.
If you think about the fact that village pump is used daily, for example, the introduction of such license would prevent further violation quite effectively. So are the increasing amount of Template: texts and wikiproject templates that will be developed in the future. Some materials are sent to meta. And many language-wikipedias perhaps translate some of key policies from English Wikipedia first.
(Besides, Japanese Wiktionary has just started, making it even more reasonable to dual license it.)
While I am not intended to push the idea at this point, it should happen sooner than later, and being late is better than never, it seems.
(Disclaimer:I am not a lawyer.)
Regards,
Tomos
_________________________________________________________________ Stop worrying about overloading your inbox - get MSN Hotmail Extra Storage! http://join.msn.click-url.com/go/onm00200362ave/direct/01/