Center trace between two pads
If you know the x-y coordinates of the two pads, then just draw a trace and then edit its properties so that it is located equidistant from the two pads.

Now if the pads are on grid (e.g. if you are using an 0.05" grid and 0.1" pitch ICs), then you simply draw the trace on-grid and it will automatically be in the centre. This is why trying to stick to grids is a good idea - though I accept it falls down a bit if you are mixing SMD ICs with different pitches.

Beyond that there is no way that I know of to do it directly. Eagle doesn't have a way to align objects to other objects (or distribute equally). If you don't directly know the x-y coordinates of the pads, you need to open up the package (in Eagle 6.x and newer you can right click -> edit package), find the coordinates in the library, and then offset them by the component origin in the board. Then edit the trace properties to be equidistant.