From what i have heard no server mergers have ever taken place. Im also not a programer, but i do some work within SQL databases. I would think the problem that they would have with server mergers is duplicate toon names on each server and how to get the data from getting corrupted with the other users data. Sounds simple enough in theory to just change toon name with a prefix of the server, but becasue the information attached to that toon (gear, stats, etc) is sorted by that toons name or ID, then it would be a larger task to rename all data associated to the toon/ID, that way no data is duplicated but still associated to the correct toon/ID. It could be as easy as adding a single server value to every entry, but i doubt that is the case or it would have already been done.
Then add to that the massive amount of toons that are not active. You are looking at millions and millions bits of data that needs to be transfered over. If it is not done correctly if will fail miserably. So I am one to say stay patient as it can be done, but will not be a short process. I figure they will start with lesser populated servers migrating together 1st as they are easiest to rollback if things do not go well.
I could be completely wrong, but from what i understand of databases, it can get tricky to get it done properly.