i do not think it will be available to the 8900 it, it would take a lot to get that to be perfect on a install for that phone.
if there is one, it wouldn't be only the trackpad itself but also you have to install a new casing and board to accomodate the chip/shape of the trackpad...but i'd love to have it installed on my phone )
I do not think curve 8900 will ever have a trackpad or be able to buy one for it the device is built to support a trackball, not trackpad. You would have the modify the entire device. All newer devices have trackpads the only curve with a trackpad as you may know are the 8500 series.