I've found in Suse that the dhcpd also creates a default route, in this case to the router. I assume you're relying on your router to get the name servers. I can't remember the syntax but I think there's a route config file for each device. Something like ifroute-eth1 for your wireless card. You may need this config file if you just want to use ifup. I may be wrong but just another angle to consider.
Just looked up the specifics. Here's an example:
file: /etc/sysconfig/network/ifroute-eth1
entry: default 192.168.1.1 0.0.0.0 eth1
hth,
jason.