使用iOS4，在睡眠时Wifi连接无法保持活力

Knowing that iOS4 support wifi stay alive in asleep is really happy thing. However I got some different experience: I've got 2 iPods, iPod 2nd gen & iPod 3rd gen. Both updated to iOS 4 beta. iPod 2g can stay wifi on in sleep while iPod 3g always disconnect wifi in sleep. I ping both of them while they're in sleep mode, only iPod 2g keeps reply echo.Knowing that iOS4 support wifi stay alive in as