I use the Apple earphones w/mic, and that allows me to use a voice-over-IP softphone on the iPod, as well as Shazam and other apps which use a mic. They work reasonably well, but I might also get the Griffin product too.
I've gone beyond what most VoIP people do, and I run my own VoIP switch (called Asterisk, for those who might know what it is). This works as my own PBX in my house, and I can call out using my landline. So, as long as I am somewhere with Wi-Fi, my iPod Touch works like a cell phone.