I have the same issue, but it didn't bother me until I saw your post.
I made the reservation on March 29th and still haven't been charged either. I got the email with the reservation number without any problem. When I was in Japan last year, I was charged a month after some purchased were made so I wasn't really anxious...
But now that you mention it... Well, I'll try to call reservation center tomorrow morning (I live in France, so I won't be able to do it sooner) and see what kind of answer I'll have. I'll let you know !