Sprint has released some information about an actual time stamp today when Mango will begin rolling out for the HTC Arrive on their community forum.  1pm CST (just about an hour away).

We only recently covered a representative conversation between the carrier and our Seth Brodeur confirming that today's the day and it will take a few days for the rollout to complete. To those on Sprint, if you've received notification and have updated please do let us know in the comments or via tip!

Source: Sprint Community, thanks Jetface for the tip!