It's basically common knowledge, check http://www.batteryuniversity.com/ and Wikipedia, among others, for more info.

Li-ion batteries have a shelf life (if you don't charge/discharge them at all) of about 2 years, depending on the charge state (50% or so is best) and temperature (cooler is better) or a cycle life of typically 300 full discharges' worth, depending on depth of discharge (really deep discharges are much worse, shallow to moderate are similar). So typical service life is a little over a year, more or less. The aging/wear manifests itself not only as reduced capacity, but also increased internal resistance; this means a worn out battery may support idle current for hours, but die in 10 minutes of high-current usage (like Skype, which is running wireless, speakers, and heavy CPU all at once). No, that's not a problem; in fact, the best (practical) thing to do is keep it on the charger all the time. Of course, the best thing would be to float it at 60% or so charge (for maximum shelf life), and run everything off wall power, but that's not really an option, so holding it full and running everything off wall power is second best. Holding it constant at any state of charge doesn't add to discharge cycles, while unplugging it and then replugging it when it runs low does.