I got called into Ambroses's office and like the bug said he had nothing to say. But I spoke to him anyway, and he said "Young wizard, come and see me at once. I have urgent news.....about Celestia!" I'm only a level 40 Life Wizard. does this mean I am going to Celestia early?
You got the call from Ambrose because of the known Bug.
My understanding is that anybody who visits Ambrose for the first time since Celestia went public, summoned or not, will be told about the existence of Celestia. Professor Ambrose is just letting wizards know of Celestia's existence, since it's relatively new (and not everybody watches the forums or visits the websites).
When you've reached level 48, finished "The Final Countdown" (defeated Malistaire), and (I believe) also done your level 48 spell quest, you'll get the official summons to go to Celestia, although you may have to do a few preliminary things to get the key.
Might be a glitch. Ambrose has been yelling at me for six levels straight, but never had anything for me when I showed up.
He's probably just trying to confuse you - the old coot is scared that we decide Wizard City would look prettier if it was, say, on fire, and at that point have become so powerful that he can't stop us. By constantly getting us to show up, he can continually decide just how strong we are, and distract us with shiny rewarding tasks instead of giving us time to think about, say, how Ravenwood would look if it was submerged.