Re: What is Recommended Practice for Long Term PlayBook Storage
You could leave it plugged in while charged, but there's no particularly good reason to do so, and you're merely increasing the risk of damage either from some electrical surge, the charger failing, or just having the thing being on. The charger doesn't "trickle charge" in quite the way most people understand that term, which is fine because you don't generally want to do that with lithium anyway, as I understand it. Rather the charger charges it up fully, then stops feeding current to the battery and supplies all the power the device requires. That means the battery is practically disconnected, and unused, and (based on the long-term test I once did and documented in these forums) it will take many many weeks before there's a perceptible decrease in charge. One can assume that at some point the charger would kick in again to top up the charge, but it appears it would take months if not longer for that to occur.
Shutting it right down, on the other hand, is a good thing to do. For maximum preservation of the battery's life, this should not be done at 100%, but closer to 50%. Manufacturers typically charge to around 40% before shipping the batteries or devices, as that's (presumably) been identified as a good tradeoff between safety and health of the battery.
I did another test once (also documented somewhere around here) and turned the device off for many weeks. It discharged so little in that time that when I extrapolated, it would take something like (going by memory here) over two years before the charge would drop from 100% to anything close to 0%. Or maybe it was just over a year... but that was my own device. There are differences between PlayBooks (at least several different revs of the hardware) and it wouldn't do to apply my experiences blindly to your situation.
I suggest charging it to 50%, or discharging it to that. Make a note of the level and, if you have Battery Guru, the voltage. (Of course, Battery Guru does that for you in a file you can export as CSV, if you like.) Then power it right off. If you have a chance, turn it on again in a month and note the change, do a bit of math, and extrapolate how long you've got. It's likely a very long time, far more than the 3-6 months you need.
(Lithium batteries have a life that is in part dependent on their average charge (or something like that), which is why you should prefer to store it near 40% instead of 100%.)
Worth noting that the single worst thing you could do is to let it go well below 0% (which is to say, below the approximately 3.4V which is where 0% is supposed to be). This is about the only thing you could do to actually damage the battery permanently. If you aren't certain about all this, you might want to err on the side of caution and just turn it off at 100%, accepting the likely very small decrease in battery life in return for greater peace of mind. (Personally, I'd still peek at it after a few months just to see how far it's gone, but that's just how I am.)