Hey guys, just wanted to let you know that I just successfully replaced my trackball in my Eris with a blue blackberry replacement. If you're wanting a different color for it but are bummed that it doesn't have all the multicolored LEDs like the nexus one, this is another option. Doing this WILL void any kind of warranty/insurance you have on your phone and I take no responsibility if you screw anything up. Just get a replacement trackball from DealExtreme. Link posted below.
Then, all you have to do is remove 4 screws from the back of the phone (one under a tiny sticker) and then 2 more inside the phone that are holding the charging/usb port in place. Gently lift the circuit board and pull out the old trackball. Pop the new one in, screw everything back together and you're good to go. Keep in mind that you won't be using the chrome ring surrounding the new trackball, just the inside part of it that is the actual trackball. It's not a very difficult mod at all, and it may even work on other phones (not sure). Anyway, Good luck guys!!
Edit: Here's the link to DX: http://www.dealextreme.com/search.dx/search.trackball
Edit: Here you go guys:
^^ The picture doesn't really show the glow of the blue :( ^^
Hope that helps!