For lithium-ion, it is actually now considered bad practice to completely drain the battery before recharging. NiCd and NiMH batteries need this to be done to minimize their so called "memory effect" - not so for Lithium Ion. In fact, it will actually be harmful for Li-Ion batteries.
I quote you this paragraph from this link
A lithium-ion battery provides 300-500
discharge/charge cycles. The battery
prefers a partial rather than a full
discharge. Frequent full discharges
should be avoided when possible.
Instead, charge the battery more often
or use a larger battery. There is no
concern of memory when applying
unscheduled charges.
There are many other sources online that support this statement, but I know this from my long-time exposure to battery conditioning due to certain hobbies (e.g flashlights).
From Wikipedia's entry of Lithium Ion batteries :
Lithium-ion batteries should never be
depleted below their minimum voltage
(2.4 to 2.8 V/cell, depending on
chemistry). If a lithium-ion battery
is stored with too low a charge, there
is a risk that the charge will drop
below the low-voltage threshold,
resulting in an unrecoverable dead
battery.
This might be relevant as well: http://www.lifehacker.com.au/2009/10/an-exhaustive-guide-to-saving-your-smartphones-battery/
– AnC – 2009-10-30T20:27:14.130