Pseudonymity, for those new to it, is the use of a semi-permanent, but incomplete or false identity. For instance, in many online communities, I'll just go by my first name, with a specific Gmail address so that people can distinguish me from all the different Andys out there. I have different pseudonyms in different spaces; in some of them, people who already know me know my pseudonym, but strangers don't.
This is a pretty common practice. It's better than anonymity for community building, as I've noticed that when people feel truly anonymous, they tend be less courteous and more inflammatory. But as the Michael Hitzlik furor has pointed out, people can still abuse pseudonymity; how do you know when 50 people are really only one person?
The answer is pretty simple. A pseudonym should be much like a nickname - you can have a different one for each group you hang out with, but it still doesn't let you pretend to be 4 or 5 different people at once.