I started modelling the mouse today, the bottom of the mouse was fine to get. I made the bottom of the mouse using the spline tool, then mirroring it to ensure the shape of my mouse was symmetrical. I then used the side view picture supplied by Dan to create guide curves, to create the surface loft; (but we will get to that later) which i then mirrored again so the shape would be the same on each side. I created 12 guide curves for the bottom half of the mouse ( 1 on the front and the back and 5 on each side).  I used the surface loft tool to create the bottom half of my mouse; then used the planar surface tool to fill in the bottom. I then use the face fillet tool (with continuous curvature) to create the fillet between the side and the bottom at a radius of 2mm.

The complete bottom of the mouse: