As expected, and confirming my similar tests from a few years ago using Series 80 on the Nokia 9210i, Symbian OS is 100.00000% inherently stealthy when online. Look at the screenshots below, taken when online on the latest version of Shields Up! Symbian OS (in this S60 2nd edition, OS 8 guise) achieved a perfect score, with all 1056 tested TCP/IP ports resisting all attempts at unwanted communication from the outside world. The column of green squares goes upwards for all 1056 ports by the way, and it's green all the way up to square/port 1.
If you want to try the experiment for yourself, just head over to http://grc.com/ and click on Shields Up!, then choose to 'Probe all Service Ports'. The test will take a couple of minutes max. The longest wait is for your mobile web browser to render a page with 1056 little graphic squares 8-)
Update: after comments about some of the stealthiness being because I was connecting over GPRS, with my network provider's systems acting as a firewall, I tried my Nokia 9500, online over W-LAN, on the same Shields Up! test. As you can see below, the ports all check out perfectly again, with no weaknesses, although the Shields Up! test did report that the 9500 was responding to Ping requests. Not a security problem in itself though.
So, I repeat. Symbian OS doesn't seem to have any need whatsoever of a firewall. Security firms, please take note and stop trying to scare us (and make money in the process) again. This has been a public service announcement.
Steve Litchfield, for AAS and 3-Lib