According to PW Insider, Tony Schiavone has signed a deal with All Elite Wrestling. Dave Meltzer said this might be the case recently as well and WWE was also reportedly interested in the WCW Nitro announcer (they were trying to sign him up to a deal as late as this past Wednesday).
However, it's said that he'll continue working with MLW as well so, unlike WWE, AEW probably didn't want to make him sign an exclusive contract.
We don't know what he'll be doing with the company but he's been appearing in AEW video packages for a while now and it's worth noting that he was once on TNT as the lead announcer for Monday Nitro when WCW was still on the air.
Jim Ross is going to be AEW's play by play announcer so it will be interesting seeing whether Schiavone joins him or if he'll have a backstage role.