Lithium ion batteries can still be overcharged... but you have to keep in mind that when it's connected to the charger, it's not taking a full 800 mah @ 5 volts... when it hits fully charged, it stops charging, and then it just takes little boosts of juice as necessary, I've noticed at about a 300-400 mah rate.

Leaving it on the charger should cause it no harm. Try to keep it topped off, and just drain it fully every once in a while.

An additional note, you can DISCHARGE the battery too far to be charged again. The lower the charge on a lithium ion battery, the more energy is required to boost the charge again.

I'm just going to pull numbers out of my *** here, but this is for demonstration sake:

Say you have a 1000mah battery, so 10% is 100 mah. Simple enough.

To charge it from 90% to 100% could take, say... 110 mah.
To charge it from 10% to 20% could take 140 mah.

While a 100mah charge gets stored to the battery either way, it consumes more energy to charge it, the lower it is.

Now, I'm very positive those numbers aren't correct in any form, I'm saying again, it's just for demonstration's sake.

What lwrdchvy2 is saying in his first post about full cycles being better applies to other battery types. Lithium ion batteries do not display memory characteristics. They -can- be overcharged, and they -can- overheat. Heat and number of charge cycles determines the lifespan of a lithium ion battery. You don't need to worry about an overcharge though because the phone manages the charge. It seems to stop charging when the battery voltage is 4.2v. You could possibly overcharge it by lowering the temperature drastically so its voltage drops, then add more charge... once it warmed up, it would then be overcharged... but who really charges their batteries in the freezer?