They are many reasons why normal bridging will not work over wifi (at least not unless you are using ad-hoc mode with weak or no encryption). Among others, in infrastructure mode the mobile stations use their MAC address to register to the base station, so bridging traffic from other MACs will confuse the base station. It may cause problems with WPA encryption as well.
If you really need (layer-2) bridging, you should look at WDS or 802.11s mesh networking. Both address these specific problems, and they are supported by the drivers using the new software stack (mac80211). I'm not sure if Ralink adapters are supported yet.
Another possibility is to split your address space in two, and use tricks like proxy-arp and brouting to make a hybrid bridge/router. This will make the hosts believe they are on the same network segment, while preserving normal MAC operation.
But really, if all you want to do is to share internet connectivity (in the client-only sense), the easiest route is to use NAT.