I don't think they postponed the discount validity to June just because the availability of the tablet will be June!
They postponed it just to be sure everyone will have a sure chance to use it.

I am not a Nokia employee (unfortunately), but I am sure you just have to wait until the letstalk.com shop ( that is NOT Nokia, AFAIK ) will get it in stock.
How much that can be ?
Days, few weeks.. not months!