Not strictly true...
Mobile phone batteries now use Lithium Ion, which altho they are fantastic for longevity, have a limited "number of charges" before they die. It doesn't really matter how you charge them, they will die after about the same amount of charges.
Also, the "memory effect" isn't that the batteries believe they are flat - it is due to the chemical process that occurs during charging/discharging. Hydrogen gas is a by-product of the charging process, and one of two things can happen:
1) The battery is charged too quickly, and the hydrogen gas "vents". This means that during discharge, you have physically lost the ability for the reverse chemical process to take place, ie, you capactiy is immediately reduced. There's is nothing you can do about this once it happens - the only way to prevent it is to avoid venting the cells during charging by charging slowly, or stopping once a tell-tale "full" condition is detected. This is why good "fast" chargers cost a lot of money. I've never seen such a charger outside of specialist applications - don't expect to find one in Maplins.
2) The buildup of hydrogen gas reacts with the electrolyte used in the cell (differs depending on the type of battery) and creates a corrosive substance, which corrodes the plates (annode/cathode...can't remember which).
Surface area of these plates is proportional to the capactiy, so corrosion is NOT good. This CAN be counteracted to some extent, by deep-cycling the batteries (ie, completely discharging them and then charging them fully), but this can also lead to "reverse polarity" if not done properly.
NiCads are most susceptible to this type of memory effect, and NiMHs are less so. LiIons are gonna wear out way before any memory effect becomes noticeable (they're not high drain enough) so people who do this "deep cycling" on their phones have been told a load of cobblers by some muppet with all of 20minutes training from a "phone consultant".
