So your saying use a large capacitor (like I said), but put a resistor in between the usb 5v rail and the caps/rest of the mouse. I tried a large capacitor, and it helped, but not as much as I would like. I bought a 100 microhenry choke, and it made little effect (the guy at radio shack said to use it). I think someone posted on here to use one rated for 5.6 milihenries, so when I get the chance I'm going to return this choke and ask the guy at radio shack to order the right one. I've noticed that the caps will affect the low frequencies more than the high frequencies, while the inductors will do the exact opposite. The noise my mouse generates is quite high in frequency (I know they are over 8 khz because my mom cannot hear the noise, and I know her hearing cuts out at 8 khz), Thus my reasoning would suggest an inductor would be more suitable in this situation. I'll report back when I get the part.40-50mA average when the LED is on (when the mouse is moved).
I would try a large cap inside the mouse across +5v and GND. Like a 470uF electro, and a 0.1uF cap in parallel with it.
Then I would decouple the incoming USB +5v rail with a resistor. you can afford to drop about 0.3v and since the mouse uses about 50mA tops a resistor of R = E/I;
R = 0.3/0.050
R = 6
(so use a 5.6 ohm 1/4 watt resistor).