Okay, so they didn't invent it per se, but Fangraphs today started carrying the Ultimate Base Running stat, which attempts to measure the runs generated or lost via the skill of base running.

And the Cubs are the worst in baseball at it (sort by BSR to see the rankings).

In fact, they have 3 of the 10 worst baserunners in baseball of players with 100 or more PAs (Fukudome, Ramirez and Soriano are 5th, 9th and 10th respectively).

Here is Fangraphs' primer on UBR.  This excerpt sums up most of the situations where a player can get a positive or negative ranking:

1) On a hit, advancing an extra base, not advancing an extra base, or getting thrown out trying to advance an extra base, as long as no other base runner is blocking an advance.

2) A batter getting thrown out trying to advance an extra base on a hit (if he successfully does, we don’t know it, as he is simply awarded a double, for example, on a usual single where he advances an extra base).

3) On a hit, the batter advancing, not advancing, or getting thrown out when a runner is safe or out advancing an extra base.

4) Trailing runners advancing, not advancing or getting thrown out when a leading runner is safe or out trying to advance an extra base on a hit or an out. This is basically lumped together with #1 above.

5) Runners trying to advance on fly ball outs – i.e. tagging up.

6) As mentioned above, on ground balls to the infield, runners on first staying out of the force or DP at second base, whether the batter is out or is safe on a FC.

7) Also as mentioned above, a runner on second advancing or not (or getting thrown out) on a ground ball hit to SS or 3B.

Also worth noting is that the Cubs have been in the bottom 3rd in baseball in baserunning every year since 2006.