iface lo inet loopback
iface eth0 inet dhcp
iface wlan0 inet dhcp
wpa-ssid "YOUR SSID"
wpa-psk "YOUR PSK"
iface default inet dhcp
This is all I did to configure my wifi. But i didn’t use the retropie-3.0 sd card image, I installed retropie binaries with the setup script on a fresh raspian from 2015-02-16.
Does your wifi dongle work? Check with iwlist wlan0 scan. Have you tried using a static ip?
did you reboot your router just to make sure ? I know it’s obvious, but worth checking at least.
DHCP does work on the latest image as I use it myself. Nothing specific to retropie has been changed in regards to dhcp from raspian. Could be some new incompatibility with latest kernel and your wireless chipset of course.