Hi,
Same problem, bar get's stuck at 90% on iPad, while on iPhone it works.
There are several forum subjects dealing with this matter, I have tried all tips (switching off/on, change DNS to 8.8.8.8, delete all old conversations, etc.) but nothing helped.
Then I tried something that sound ridiculous and unlogic: Although we do not use 3G often for the iPad and it is inactive, we changed the sim card and now it works!
I agree this is strange, because we're only using WiFi, but it seems that the 3G models still check the installed sim. We travel between different countries, and with a Maltese sim card iMessage does not work, but with a Dutch or Italian sim installed we can send iMessages over WiFi. (so 3G and cellular data are switched off)
Weird...
I guess this won't solve the problem for everyone, but maybe some for some of you this will work...
Good luck, Dan