I posted this some while ago in another similar thread, but it is probably worth reproducing it here.
Li-Ion (LiPo, LiCo, LiMn, LiFePO4 and other Li chemistries) will have a longer life if not allowed to discharge completely. Modern devices incorporate sophisticated protection circuitry to avoid serious damage from over-discharge, but bumping down against that limit too often will not do the battery any good.
The important thing is not to leave the battery in a fully discharged state. Recharge it as soon as possible after discharge. If you have to store the device unused for any protracted length of time (1 month or more) do so with the meter reading about 80%.
The battery should last around 1,000 full charge cycles - that is, complete discharge and recharge every day for 3 years. Few people will achieve such a heavy use. A part cycle does not count as a full one, so don't be afraid to top it up daily, even from 70-80%. LiPo batteries have no memory effect, and prefer being kept topped up.
None of us, even heavy users among the very first to get iPads, will be anywhere near exhausting the batteries yet. In 3-4 years' time, if we have not upgraded by then, the symptoms of exhaustion will be a failure to hold charge; the battery will charge up to 100% but then drop back quite quickly (within 15-20 minutes) to a noticeably lower figure even when it has not been used. When it can no longer hold 80% charge, it will be time to retire it.
By then, users may notice the iPad becoming warm (or even quite hot) during recharging. Heat is not a good sign; the problem will increase as the battery's life approaches its end, to the point where it becomes dangerous. By then the device is likely to exhibit other signs of malfunction. Further persistence at that stage would be a bad idea as a battery that overheats can eventually suffer thermal runaway, at which point it will go off like a firework and vent with flame, producing extremely toxic gases in the process.
However... the greatest likelihood is that the battery will simply just gradually fade out, with none of the drastic worst-case scenarios occurring; and it isn't going to happen to any of us for quite a long time yet.