I'll file this under opinion since there is not one inkling of confirmation from anyone except the janitor's brother's sister-in-laws's cousin that cleans the building next to the local Sprint Repair Center here, but...
May 14th - Repair Center update only - May 21st OTW update available (with SDX and everyone else ripping it out and posting it online by the 15th for download...)
I doubt that we will see an OTA for something this major - although crazier things have been done. You are going to have thousands of people that have completely different phones after this update so going in-store only makes sense.
As far as Rumorsville continues: The Hero update was to 'blame' for the delay in the Moment's launch. Apparently, Sprint has had the update ready for the Moment for about a week now, but due to the issues with the Hero's update, they did not launch. The Hero was 'due' on/around April 9th with the Moment to follow in about 2 weeks. There was a major issue with the Hero that has delayed it until <European update in June?> who knows when, but it's not looking good for early May...
Of course, Sprint still has to release a date, but they have told their customer service agents to commit to "Early May" as a time to update to 2.1.