Well, it's not in stealth mode, because it responds to ICMP echo (ping). The test results you're seeing on GRC.com are likely viewed while sitting on a DSL/Cable modem, which is using NAT in and of itself (i.e. you've got a private IP address, your modem has the public IP address and routes traffic to you acting, in effect, like a router).
But I was surprised to find that you are somewhat right. I opened up my MacBook Pro's ipfw, running 10.6.2 and ran a port scan from my desktop PC running Windows 7. I used a couple of different products to do the scan and in both cases, found that no well-known ports were responsive. I did not run a full port scan, since it would have taken forever (and it's doubtful that a hacker having identified the presence of your machine would waste that much time, unless it was a high-value target).
But this paints a rosier picture than is reality. Reality is that if a vulnerability were identified, running no software firewall and sitting on a publicly accessible LAN would leave you susceptible to a worm that exploits that vulnerability. Additionally, if you happen to be running a piece of software that opens ports (like an IM client, for example) and that software has a vulnerability (as was discovered in iChat back in 2007), you could also be susceptible to a worm.
So, I stand by my argument that unless there is a problem directly attributable to the software firewall, it's better to have it running - particularly on a portable machine that leaves the relative safety of a home network.
Oh and please keep your metaphors clean, this is a family-friendly forum.