The battery capacity is reduced after a certain number of recharges, but it's more really the amount of charging than the number of charges. In any case it's totally different from NiMH, not to mention NiCd batteries. Don't worry too much about charging often.

What really kills the lifetime of a lithium-ion or lithium-polymer battery are two things:
1: Deep discharge. A single very deep discharge of my S-E phone reduced the capacity from nearly 3 weeks to 2 days.
2: Staying at 100% charge and being too hot at the same time.

No. 2 is not much of an issue for the N900, it can be a big one with laptops though (they can be quite warm while staying on the mains all the time).