Sprint has implemented a vacation freeze for all front-end employees for the month of May in preparation for the Pre launch.
The first window is based around receiving Pre units late April to early May in preparation for a preliminary May 17th launch. The second window postpones the launch date to June 29th if the initial shipments are deemed too low to begin initial sales in May.
A May launch date would beat the new iPhone to market whereas a June 29th launch date would be closer to the expected iPhone release date. Last year the iPhone 3G was announced on June 9th with a July 11th release date.