Yep - I had a letter request and phone interview back in march. Apparently they work in batches of reading 100 plays at a time, and picking a few to request letters from, so your play was likely in the last batch.
The timeline is a little confusing. The email from the admissions people said they would pick the semi-finalists by may 15, but on the phone it sounded like they would have the finalists by may 15, and then give Chris and Marsha a month to read/pick them.
Or it could be that everyone who gets a letter request is a semi-finalist. In the past it sounds like they've generally had 20-25 semi finalists, and then of those, 10-12 are finalists. There are 4 spots. Though all those numbers vary from year to year.