From nookDevs

Part of the NookColor portal

An option to keep wifi awake while the screen is off is to prevent the Nook from going into sleep mode. This can be done by using an app which will create a WakeLock. WakeTimer from the market will let you do this (as will media players still playing when you shut off the screen). When the Nook is first unlocked, the wifi symbol will initially be off even though it is still active. Pandora paused is probably just as effective.

An alternative option is Advanced Wifi Lock which is specifically designed to fix Wifi issues and will automatically enable a Wakelock only when Wifi is connected and also has the option to automatically disable the Wakelock when wifi becomes disconnected (e.g. out of range of Access Point)

Beware of impact on battery life. A Wakelock will prevent your device from entering low power mode.