I'd say that they will NOT all get the update at once, based on the experience with the NODO update. Microsoft will release it to all the carriers at the same time, but they will each decide when to push it out to their customers.
I too would like to know if vzw will get any new phones with Mango at launch. Not to mention wishing I knew when that launch would be...
All phones will get Mango. Carriers will test the update like with NoDo, but Verizon is typically pretty good with updates. Most carriers will see it within a week or two of each other, with AT&T probably being last like usual. So yes all devices will get it, not at the exact same time, but pretty close.
Microsofts first release (NoDo) didn't go as smoothly as they would have liked, and are guaranteed to be pushing the carriers hard for Mango since not only is it a huge update which will coincide with a large scale advertisement push, but will be competing with iOS 5. I think this one will go much more smoothly as NoDo was a learning experience for the new platform.
Nobody knows what phones are going to be released, nor to which carriers yet but you can count on phones going to all current carriers including Verizon. My guess would be an LTE phone of some sort from HTC at the very least. I also predict Mango will bring another sprint device which will be nice for those folks.
If you read this thread: http://forums.windowscentral.com/win...ion/178821.htm there are some things I explain here but long story short Microsoft learned from the mess that was the NoDo update and will have Mango ready for everybody at the same time in the fall. Some exec from a Microsoft branch in Europe somewhere said that it's highly likely it'll be rolling out around mid October.