To shill my DNB FAQ: http://www.gwern.net/N-back%20FAQ

As to temporary: if it's temporary, it's a very long temporary. From personal experience it takes months for my scores to begin to decay more than a few percent, and other people have reported scores unaffected by breaks of weeks or months as well.

The more serious concern for people who want big boosts is that looking over the multiple IQ before-after reports I've collated, I have 2 general impressions: that DNB helps you think quicker, but not better, and that the benefit is limited to around +10-15 points max.

(On a personal note, ZoneSeek, if after a few weeks or months of N-backing you've risen at least 4 levels and you retake the Mensa test, I would be quite interested to know what your new score is.)