According to an announcement on the Sprint website, the HTC Evo 4G will be getting Froyo on August 3rd. They said that users will get the update in waves and all users should have the update by the middle of the month.
The users who own an Evo 4G will receive an SMS from Sprint as to when their download is ready. They will then receive an over-the-air update to their operating system automatically. Those who can't wait will be able to download the update manually by selecting HTC Software Update in the System Updates section of the phone.