This is a broken patch to a broken system. The idea of karma is to indicate users who've contributed more value (and are thus more interesting) to the system. The problem with setting a cap is that you've now got a ceiling against which more and more users inevitably pile up. As the SlashDot implementation of karma is a simple summation of recent moderations, good users' karma inevitably tends upwards. Unless there is a way to "evaporate" karma over time, the only thing which differentiates different users is the rate at which they pile into the ceiling.
These are issues addressed specifically by KuroShin and the ScoopEngine moderation system. -- KarstenSelf