The N900 is using dnsmasq for the local caching. The manual page for that would suggest that adding "clear-on-reload" to the /etc/dnsmasq.conf file will force it to flush the cache whenever the DNS servers change (the manual page only mentions /etc/resolv.conf though, so it's not clear whether that setting also applies when you're using alternative config files).

I'd suggest adding that line and rebooting (to force dnsmasq to reread the config file), then trying it out.