Thanks for this Patrick - I agree with everything you say here.
To go back to the original questions - paraphrased with my responses:
1) Is there a developer's list?
2) Is there a some way to get a feel if/when there will be an updated version of OpenRefine released?
Not really - while I'm inclined to be slightly more upbeat that Thad in his reply, there is no doubt that the amount of resource available to OpenRefine is minimal and getting a release (or a new release candidate) out is difficult and the timing unpredictable. There is, of course, no guarantee there will ever be a final 2.6 release (although I feel it will get there eventually). However, on the plus side we are still seeing a low level number of commits and PRs in the code base - so while it may not be changing quickly, it is still alive as a project - not least because of the active user community as can be seen on this list
3) Is it worth investing time in learning OpenRefine?
Absolutely! Even if there is never another release of OR I'd still recommend investing time in it - if you work with data I'd say it is a key tool in your toolkit. When doing a 1 hour intro to OR I've had people say to me "this would have saved me hours" - so I believe that for people doing this kind of work, even if OR was only going to be viable for another 12 months, it would still be worth learning. However since OR 2.5 (released December 2011) still works 5 years later without any major issues I think it is a reasonably safe assumption that a version of OpenRefine will continue to work for several years even if all development stopped completely today.
So - I'd recommend investing now and starting to use OR as soon as you can (assuming it is relevant to your work!).
And finally to echo Patrick - thanks to those who have worked, and those who continue to work on OpenRefine - I use it almost everyday, and I find it not only helpful, but a pleasure to use :)
Owen