Update: The Microsoft Windows Phone History page was updated as of June 28 to include mention of a new update -- which is seemingly Tango. That update, 7.10.8773.98, is listed as including new multiple message attachments for text messages; SIM-card contact exporting; and "other quality improvements." The update is listed as being "not available in all markets or for all phones."
The Tango updates that all Windows Phone users are expecting may include wifi tethering ("Internet sharing"). Wifi tethering is one of the features some thought to be part of the Tango updates, but it's really more of a hidden Mango feature. The Nokia update page makes it seem "flip to silence" (flip your phone over to silence incoming alerts and calls) may be another of these Tango updates that some/all current Windows Phone users will get.
Last week, Microsoft officials made it official that existing Windows Phone handsets, including all of the currently shipping Nokia Lumia models, will not get the Apollo operating system, expected out later this year. Instead, existing Windows Phone users get the Windows Phone OS 7.8 update, which includes only UI changes including the addition of a third, user-selectable tile size, plus the removal of the right-hand gutter on the Start screen. (I know some believe there will be other Apollo features in the 7.8 update, but I asked again to be sure this week and was still told by a Microsoft spokesperson this would be a UI-only feature update.)
Update: What about Lumia 900 users? When will they get Tango? It seems from this Nokia link (thanks @ZuneTracks) that Lumia 900 and Lumia 610 users already have the Tango updates to Mango. However, @ZuneTracks said it's Canadian Lumia 900s that have Tango, but U.S. Lumia 900 users are going to have to wait for the Windows Phone 7.8 update to get these features. For the record, Microsoft officials won't say when the 7.8 update will be out. I am checking with Nokia to see if anyone there will verify this information.
Update No. 2: This just in from a Nokia spokesperson:
"The Nokia Lumia 900 supports wi-fi tethering. AT&T requires a separate data plan to support the feature. The flip-to-silence feature for the Lumia 900 is planned but we do not have a date for that yet."