I had the exact same issue when my co-woker's try to connect their iPad/iPhone to my N900's wifi either from Joiku Spot, or the mobile hot spot application from the extras repos.

As near as I can tell (cause the terminal on the iPhone OS blows), it DOES get an IP address from the N900 over Wi-Fi and it does get nameservers, so DHCP is working on the Nokia side.

However, it can't get any packets in or out any further then it's default route.

It's almost like there's a proxy setting on Apple devices that is incompatible with this method of getting online. Not really shocking given the numerous methods of invisible control I've come to expect from Apple engineers.

They're just evil.