Inside there is, well, not a whole heck of a lot. A large spacer prevents the user from pressing down hard enough to damage the small logic board. That board is connected to the trackpad itself via two "ridiculously thin" ribbon cables, which are glued into place. They used a hot air gun to soften the adhesive. So, tread very lightly if you plan to remove these.
Speaking of the board, the gang found that it holds a Broadcom BCM2042 for Bluetooth connectivity and a Broadcom BCM5974 touch screen controller chip (the same on used in the iPhone, iPod touch and MacBook Air). A SST 25WF020 provides 2 Mbit of serial flash memory. On the back, there's just a whole lot of glue.
Here's a bit of good news: the Magic Trackpad's battery is user-serviceable! Just good luck getting to it. It's also notable that pressure applied to the surface is translated to the feet, which in turn presses on a plate attached to the chassis. That place squeezes an electronic mouse button switch. In essence, as pointed out by Macworld, it clicks with its feet.
Thanks to iFixit for another well-documented tear down, and for sweating the (very tiny) details.