Hey guys... I'm thinking about buying the AKG WMS 40 Pro instrument set, but then I had a thought- with so much of the noise reduction relying on the ground connection from the guitar via the amp and into the ground lug on the wall outlet, what does that mean if you're wireless?
Your premise isn't a correct one. Your guitar is internally grounded - that's why it has the ground wire connected to, say, the claw on a Strat or the bridge on a Gibson.
The ground on the amp is for the amp's circuitry and doesn't effect the guitar's ground in any meaningful way unless there is a ground loop or lift.
^Not really, everything in your guitar's internal grounding is connected to the ground on your output jack, which sends it via your cable's shielding through the amp and into the ground. This is the reason that poorly grounded guitars will produce a hum when you don't touch the strings- because when you do touch the strings your body becomes the ground connection.
Just because they're connected doesn't mean that they rely on each other. The guitar's ground has to be connected to the amp's ground because you couldn't have a circuit otherwise, not because the guitar needs it.

Touching the strings to reduce noise actually proves my point better - if the guitar relied on the amp's ground, you'd never have that problem unless the output jack were bad. Instead, you have the issue when the guitar's internal grounding is insufficient.
^They do rely on each other. Listen (or rather- read, actually), there is no such thing as "internal grounding" because the electricity can't just magically vanish out of the circuitry without a wire to take it away.

And no- the issue with touching the ground proves nothing in your favour. The reason the hum vanishes when you touch the strings is because until you do, the noise can't get to the ground on the wall due to a faulty connection.

The hum appears because the strings, bridge, metal casing of the pots, and actually all the metal parts of the entire guitar acts as antennae for background noise (60Hz hum, CRT screen noise, etc.) and without a solid ground connection to drain this electricity, the electrons will bounce around your hardware and the non-effective parts of your electronics like moths around a light bulb, and this "bleeds" into your actual guitar signal.
It's not vanishing. It's referencing. Lots of things are internally grounded - your ipod, your cell phone, the wireless unit you're talking about. A ground doesn't make electrons disappear, it just provides a reference state for the potential.

What about battery powered amps? Do you think that the negative terminal on a battery is a better ground than the sustain block on a strat? I'm not sure that it is.