I recently bought Raspberry Pi type A, i.e. the edition with only one USB connector and no Ethernet socket.
I wanted to connect to the device through a WIFI card (Edimax Nano – EW-7811Un), which turned out to be surprisingly easy:
iface wlan0 inet dhcp
iface default inet dhcp
Now simply put the SD card into the Pi and start it, it should automatically connect to the WIFI network you have described in the interfaces file.
Some suggests that more configuration is required, but that does not seem to be necessary, at least not any more.