And depending on who you talk to they will say letting the battery get lower than 15% can actually damage the battery. Just have to remember these things are small computers that can make phone calls, does your laptop last 14 hours on one charge? I highly doubt it.
I have done this research before and just reconfirmed...your worries about letting the battery go too low are likely not a real concern since manufacturers place pretty sophisticated circuitry in the battery to monitor things such as charge rate, capacity, temperature, and thresholds (the latter is where your concern is negated). When a LI battery is being charged, the circuitry throttles the charge rate and makes sure it's not overcharged, while charging fast enough to meet demand. It then "topps off' the battery and shuts charging down completely.
Likewise when the battery is being discharged through normal use, the circuitry monitors the voltage levels and signals warnings before reaching the minimums, and eventually a shut-down of the phone when the voltage drops to the threshold set by the circuitry and based on known (or thoeried) minimums before permanent damage is done. It is also generally known that unlike NiCad batteries, LI doesn't have a "memory" so full discharge followed by recharge isn't supposed to have significant impact on lifespan. Also unlike NiCad (and to some extent NiMH) batteries, LI actually likes to be partially charged often rather than full discharge followed by full recharge. So (as it turns out for me at least), it often works out well for cell phones, since it may typically be on charge at home at bed time and till the morning, then on charge in the car on the way to work, then on charge at work, then on charge in the car on the way home, and then may be on charge again several more times in the car during shopping trips or other family outings for instance, and then finally on charge again at home at bed time.
Some people carry around the travel charger, and some also carry around "battery boosters" which either have their own LI battery or may accept either AA or AAA batteries, and in some cases may take 9 Volt batteries instead. I believe there may even be "disposable" quick chargers that are not rechargeable.
Some good resources for battery information:
Charging Lithium-Ion Batteries
Lithium-ion battery - Wikipedia, the free encyclopedia
How to charge Lithium Batteries.
Some good basic summary information from Battery University:
Simple Guidelines for Charging Lithium-based Batteries
A portable device should be turned off while charging. This allows the battery to reach the threshold voltage unhindered and reflects the correct saturation current responsible to terminate the charge. A parasitic load confuses the charger.
Charge at a moderate temperature. Do not charge below freezing.
Lithium-ion does not need to be fully charged; a partial charge is better.
Chargers use different methods for “ready” indication. The light signal may not always indicate a full charge.
Discontinue using charger and/or battery if the battery gets excessively warm.
Before prolonged storage, apply some charge to bring the pack to about half charge.
Over-discharged batteries can be “boosted” to life again. Discard pack if the voltage does not rise to a normal level within a minute while on boost.
