Updated: Since one of the iPhone OS 3.x.x they made the Macally iVoiceIII not compatible, this is actually not true, if you just turned on your iPod Touch it will work for about 20 seconds, so this is a software fix from them to disallow it. As far as my investigation goes, this only happens on iPod Touch 1st generation, some people has reported that the Macally iVoicePro does do work with iPod Touch 1st generation.
When I first bought my iPod Touch I had the idea of use it with my Skype account to call to Costa Rica without having to be in my laptop, so I bought a mic for the iPod to do so.
The only problem back then was that the mic was created for the iPod video or classic, but was not tested for the iPod Touch and to make it work needed some work, the guys that made it work posted some videos on YouTube about how to disasembly the mic to hack it and then to make it work using some jailbreak applications, but I was not willing to do that back then.
The good news is that after Apple have made some software updates you don’t need to hack the mic nor jailbreak your iPod Touch to achieve this.
So, all you need is mic that is arround $30 (Macally iVoiceIII Microphone for iPod) in internet and the app Nimbuzz which is free and you can get it on the app store.
Once you have your mic with you and you installed the app you can enter and configure your Skype, gTalk or Live (Microsoft) account, plug the mic and call any of the contacts you have there (or phone numbers if you have Skype credits) and voila!!… oh, and don’t forguet to plug your headphones if you have the 1st generation iPod Touch (like I do).