...Part of the reason why we were getting good vibes from them on reunification is due to our then tentative plans to move the English Wikipedia to en.wikipedia.org. So without this small concession by us, I don't think there is a good chance for them rejoining our project. The English Wikipedia won't be harmed by being placed at en.wikipedia.org -- this is the only fair and neutral thing to do.
Poppycock. There absolutely /no/ technical benefit to this change, and it will cause massive problems by changing the URL of every page indexed by every search engine in the life of the project and increase server load. If its only purpose is to placate a few Spanish-speaking folk, then I say let them get over it. We're not here to stroke their egos. We're here to create a product that will be useful to our users--88% of whom speak English (estimated from server logs). We'll make real, substantive progress soon when we get the non-profit running and better integrate the new software. Without that, no amount of symbolic fluff will matter; with it, the Spanish-speakers will have no reason to worry about minor little nits like domain names.
Have patience, all will be well.