It cannot be your worldspot account. To reach the landing page, no radius request is made.
I've just retested a dd-wrt sp2 on a linksys wrt54gl and have the opposite problem:
It didn't work with wlan+lan and no separation for wifi: dhcp ok, I can ping 192.168.182.1 but no dns response.
I've reverted to wlan only + wifi separation and it worked well (after some time)
Then went back to previous settings, and it worked !

Then I can't make it fail again

Just wondering if there is something related to the clock. Please type date, then check that the clock is ok.
Could you please try to ping 192.168.182.1 then 88.191.29.2 (the worldspot server) from your client.
when you ping google.com do you get an ip?