What can I use those USB ports in the back of my Wii for?

USB keyboards work with the Wii Shop!

i tried charging my phone back there...not enough juice.

You're a brave man.

I'm also a member of the coney island polar bear club

Internet Channel Works, Too, With a Keyboard.

The USB ports can be used for Keyboards, USB Memory, Rechargeable wiimote dock, USB ethernet adapter, and possibly more.

NO USB Memory.

Basically what he said, except you can use some USB mice with it too..

The KB and mouse have to be corded though...not enough juice to run wireless sets.

and yeah...USB memory does NOT work in the back ports

Well, I have heard that Nintendo is planning a removable hard disk for its Wii. I am not too sure what is the volts supplied to the USB ports at the back of the Wii but the standard is 5v. There are a few "no name" brands that sell Wii remote batteries that can be charged from the USB ports. There are also Wii cooler units which actually hook up to both USB ports. I am sure Nintendo has many more little things in the works.

Maybe a Camera in the future... who knows

But add-ons for all 3 major units seem to equal off over time, PS3 has a camera, Xbox 360 has a camera, I am sure Nintendo is not that far behind. Even if their CPU is an over-clocked and heavily modified PowerPC G3 CPU (750Mhz) it has enough power to do the same multimedia the other 2 units can. So I am sure there will be many USB add-ons coming.

I also found out that Nintendo is making an OS for their Wii called the "ES"
http://nes.sourceforge.jp/
It is experimental but the possibilities are endless.

you can use the USB slots to hook up things like Rockband instruments

I bought a Wii remote recharging station that draws power from the USB and it's brilliant. I found the remotes chewed through batteries, but now they are always charged up and ready for action.