Anyone know if it's going to be an in-place update or a full wipe/swap?

The answer is ABSOLUTELY YES. See HTC EVO 4G getting Android 2.2 Froyo very soon - AfterDawn
It is rumored to be in SEPTEMBER.

we're getting it next week! EVO will be the *first* to get it!!

The Evo gets Froyo | Android Atlas - CNET Blogs

Hello Flyawaymike

That is the best news I have heard for a while !!! How do you know that we are getting FROYO next week ? What is our source ? Spill ! LOL!

Sprint announced it.

Sent from my Evo on the road.

Just saw that ! :-)
It is on PC World here - or you can Google HTC EVO and 2.2
Sprint's HTC EVO 4G to Get Android 2.2 Next Week - PCWorld
Looks like we are getting it as early as TUESDAY of next week. Woohoo. Sprint's HTC EVO 4G to Get Android 2.2 Next Week - PCWorld

Originally Posted by pkerbage

Hello Flyawaymike

That is the best news I have heard for a while !!! How do you know that we are getting FROYO next week ? What is our source ? Spill ! LOL!

Sprint announced it on their Facebook page here:
Sprint | Facebook

My apologies if this was mentioned already, but I didn't see it anywhere. I just wanted to make sure y'all were aware that if you go manually check for an update (as of 12pm CST today, I'm assuming) you can receive the 2.2 update early. This was mentioned toward the end of the official press release.

To manually check for updates Menu -> Settings -> System Updates -> Firmware update

You'll be able to "officially" update your device today or tomorrow I believe via manually rather then waiting for the notification.

I think so and hope so!
Will be great.