They wouldn't both be on the same subnet, Comcast will give out a public IP which cannot be used anywhere else in the world. Now if you are using a router, then there is the possibility of having the same subnet on both interfaces, but that is easy enough to fix by changing the subnet your router uses.
But to more directly answer your question, at least what I think you are trying to say anyway, if you have two default gateways things can get screwy. If you only want to use the WLAN to access other local computers then you don't need a default gateway on that interface (assuming that all of the computers are on the same subnet). I don't know if there's any way to use DHCP and have your computer somehow not use the default gateway it gets from the DHCP server. I would find an unused IP address on the WLAN and set it up as a static IP on your wireless card, put in the IP and netmask but leave out the gateway. That will let you connect to the WLAN, but your Internet will go through the cable modem.