Here's one answer (with multiple parts) via smarterware:
1. With Lithium-Ion batteries, Apple encourages users to let the battery run completely out and recharge it fully once/month. In between chargings, it recommends not leaving your laptop plugged in unless you need to.
2. For other battery types, the data is pretty inconclusive. Whereas neither Dell nor HP suggests unplugging to save battery life, there are many people who claim that leaving their laptop plugged in has shortened the battery life significantly.
Bottom line (as the article says): When In Doubt, Unplug. There doesn't seem to be any evidence that unplugging can shorten a battery's life if nothing else. Plus I hear it's "greener" anyway. Win-win.