what cheap sub-standard components support has nothing to do with the specification but rather the fact that they don't adhere to it.
In such a case, sometimes the manufacturer cheaps out by only using a single front panel USB header, and having a small piece of circuitry act as a hub. In this case, the power of the front panel USB is reduced in the same way as would occur with a unpowered hub, that is, < 200 mA.
The actual Front panel header on a motherboard gives 200mA, which is the full power available via the specification.
In this case your iPod may simply not charge from the front panel due to another quirk. Many front panel USB connections on PCs are built to the USB 1.1 standard; the iPod may simply not support USB 1.1.
That being said, however, I imagine a printer would gain significantly... (in fact, it does, considering now my printer and it's performance now versus when I used it with USB 1.1) and so if it turns out that the front panel connections of the OP's computer are in fact USB 1.1, or if their PC manufacturer used a cheap "I'll use a cheap hub instead of a cheap USB header" technique, then the connections on the machines rear would be best.
In fact, considering the printer isn't likely to be often plugged/unplugged, it would probably best to plug it into the rear anyway, and save the front panel connections for things like USB drives.
EDIT: ahh yes, replace 200mA with 500mA in this and my previous posts, heh.
500mA is the standard requirement. Not meeting this standard- and not being a unpowered hub - means that it does not conform to USB (strictly speaking). However the front panel ports, as I've indicated are often implemented using a low cost USB hub, whose limitations I have outlined previously.
