Este post no fórum de discussão da Apple fornece uma solução:
I have had my 2nd Gen ipod touch (32Gig). I set it up on my wireless network which does not broadcast its SSID and is 128 bit WEP (yeh yeh I know weak) encrypted. This makes for quite a daunting task when forced to type in the WEP key every time the ipod 'forgets' this access points settings.
However I did discover what seems like a quick work-around. I entered the SSID, chose WEP, and for the pass i just entered a bunch of random chars. It said wrong password then connected up to my network fine with the old profile that it appeared to have lost. Not sure if just entering the SSID brought it back to life or not, but it's better than having to manually enter those huge encryption keys.
Veja também este post do blog que aponta para o post no fórum acima: Dicas: Corrigir o iPhone / iPod touch Perdendo as configurações de Wi-Fi