if you get a response when pinging yahoo at:

ping 216.109.112.135

Then no DNS server is being set automaticly.

You can manually set it in "/etc/resolv.conf. For example:

/etc$ cat resolv.conf
nameserver 192.168.100.1

If ping fails, then the Gateway is probably not set.

Type "route" and see if there is a default gateway address that is set
to the routers address (ie 192.168.100.1 in my case!)

You could use "route" to set the default gateway, but I am not sure of
the syntax right now. If all else fails disable or remove network
manager and configure the wireless via:

System - Administration - Networking.

Wifi-radar is an alternative for dynamic wireless connection and it
works better on many machines than network-manager.