According to an interview that Microsoft developer evangelist Nuno Silva gave to Zwame, every Windows Phone sold to date should be getting the upgrade to the next "major version of Windows Phone." Silva further made it clear that he wasn't just referring to current devices, but also the first-generation of Windows Phone devices going back to the Omnia 7. Presumably, the "major" here isn't a simple tease for an interim update, but rather the version that's codenamed Apollo.
Sadly, Zwame has taken the original, hour-long interview down, but you can see the portion in question below grabbed by WMPowerUser (in Portuguese). Before you get your hopes up, it's worth remembering that Microsoft support for an update and carrier support for distributing it are two different things. On the other hand, AT&T has said publicly that another update is coming for its Windows Phones, but somehow we doubt it was referring to Apollo. We've reached out to Microsoft to see if it has anything to add, we'll let you know if we hear more.
Update: Microsoft has reached to say that "we have nothing to share about future releases," although the company does note that at the very least that "all apps in our Marketplace today will run on the next version of Windows Phone."
Update 2: Our sources tell us this is not the case, in fact the opposite is true: no Mango devices will be upgradable to Apollo.