Unfortunately, if all you have is a single mic-in jack, there's a good chance that it won't suffice for all but the simplest of podcasts where it's just you talking with no other participants or additional audio (for example music clips) being brought into the mix. I say "mix" because, once you move beyond yourself as the sole audio source of your podcast, you may find the need to blend audio sources -- a black art known as mixing. Mixing, if you haven't guessed by now, may require multiple inputs into your system. One input for one audio source. Another input for another audio source.
For mobile podcasters with notebook computers that need additional audio inputs (or in the case of systems like the iBook, just plain need one), one easy route is to find USB-based audio devices to plug into the system's USB port. There are USB-based microphones, USB-based headphones, USB headsets (combo units that offer both headphones and microphones) and USB audio adapters such has Griffin's iMic that provide additional mic-in and headphone jacks to plug in. There's also a world of more sophisiticated USB-based audio devices such as Tascam's US122 -- a $200 device that has two XLR-based inputs (often used for higher-end microphones), MIDI inputs, and and two line-in inputs. In some ways, it's better than an internal sound card and with a device like this, you can easily "mike-up" two people plus plug in another sound source like an iPod.
But, in my testing with an Apple PowerBook, USB has proven to be problematic. Why? Because USB is a multi-purpose interface rather than a dedicated sound interface and, unfortunately, in building a multi-purpose interface, certain USB implementations aren't built the way some sound interfaces are built in order to keep other sources of noise out (eg: electromagnetic interference) of your audio. In my tests, USB proved to be much more susceptible to such noise than the built-in audio jacks and the source of that noise was the notebook itself (in my case, the notebook's fan). But if your notebook is audio jack-limited as most notebooks are, then what should you do? You might consider a FireWire-based audio device. After becoming fed-up with USB, I started giving some FireWire-based audio devices a try. So far, I've tested Presonus' FireBox and M-Audio's FireWire 410. Not only do those companies have other FireWire-based audio devices to suit a range of needs and budgets, there are also other companies such as Motu and Edirol that make them as well.
Eventually, I will publish the results of my tests of these devices (I'm still gathering data, fact checking, and hoping to get some more devices in for testing). But after working with these devices, I discovered two advantages over USB. First, the audio didn't pick up any noise from the fan in my PowerBook. Second, some of the more sophisticated audio applications will recognize all of the different inputs on these units as separate sound devices that can be mapped to different tracks of audio which translates into a higher degree of versatility when it comes to not just recording your podcasts, but also editing them after.
There are two downsides. First, most notebooks, particularly ones that aren't that heavy, don't have a FireWire interface (they almost always have USB). So, perhaps this is a buying consideration when you go to purchase your next system. Second, FireWire is like USB in that the computer can share it's power source with the device. In other words, you can take your notebook computer and your FireWire device into the field and record a podcast without access to AC power (as long as your notebook's battery is fully charged). That said, it has been my experience that a FireWire device like the ones I tested will drain a lot of power out of your battery. The battery life on the PowerBook that I've been using went from a little more than two hours to under 45 minutes when one of the aforementioned FireWire devices was in use.
Oh, and one last important note: if you're hoping to take advantage of FireWire's ability to power a device, make sure the notebook you buy has a 6-pin FireWire interface rather than the 4-pin interface. Notebooks with a 4-pin FireWire interface will not provide power to any connected FireWire devices.