I've looked on the net for some info on phantom power, but basically I'd like to know what it is, and what advantages are there to using it? I'm wanting to use cordless mic's with my XL1S - can you use a phantom power XLR setup with cordless mics? If you can, are there any advantages to using it over an MA 100/200? These may sound like naive questions, but I'm still learning this stuff. Thanks for any info you can give me!

I'm fairly new myself, but I think I can handle this one.

Phantom power is carried to the mic from a specially designed XLR adaptor (see the offerings at BeachTek.com) or mixer over the same XLR wires that carry the audio from the mic to the adaptor.

The adaptor circuitry normally steps the voltage up from a 9V supply battery to 40-52V to operate mics designed for phantom power. Higher voltages are better for long cable runs--less current is required for the same power, so there is less power loss, and a smaller voltage drop. Plus, any voltage drop that does result is going to be a smaller percentage of the nominal operating voltage.

Some good mics like the AT987 can run on phantom power or a self-contained AA battery, but many high end mics require phantom power.

Not a wireless thing.

Generally the main phantom consideration with wireless is if you want to hook a handheld mic or shotgun mic to a wireless transmitter. If the mic requires phantom power then either the transmitter must supply it or you have to use a separate phantom supply first. Most transmitters only supply low voltages for powering lavalier mic elements. Some plug-on transmitters can supply phantom but many can't.
Read your mic or wireless system manuals for info on phantom power voltages. Some mics can run on a range of power, like 9 to 52 volts (or internal battery instead). Other mics need full 48 volt phantom only.
Another phantom consideration with wireless is making sure your cables from the receiver and any adapters are in good condition and wired properly. The wireless receiver doesn't need phantom, but if your cables have problems or your adapters aren't wired properly then phantom can cause crackling and popping. Often you don't have independent ability to turn phantom off. So if some of your equipment needs it, then all of your equipment must be able to handle it properly.
Like was stated earlier, phantom isn't much of a wireless issue but can sometimes be important in what I mentioned.
Phantom is much more of a regular wired mic issue. Having phantom available will allow you to use a wider range of high-quality mics versus using only battery-powered or dynamic mics.

<<<but if your cables have problems or your adapters aren't wired properly then phantom can cause crackling and popping>>>

Man o man is that EVER true.
Here in Singapore, had two different lavs, an 899 and ECM 66 that were popping and sounding tinny as hell. Spent 20 minutes changing out cables, flipping phase, transformers, etc, before switching off phantom. (mics had batts in them)
All of a sudden, simple stuff made me feel ......simple. Duh. Shoulda tried that first, but it's been a long time since phantom has tripped me.