micth: Don't know what the scientific name for it is, I've always heard it just called the sleep cycle. Basically, in normal sleep your body cycles through Stages 1-4 sleep and then REM sleep, and on average that whole cycle takes 90 minutes. Since Stages 1-2 and REM sleep are not as deep, setting your sleep schedule up with those 90-minute intervals in mind is supposed to give you much greater energy for the time invested. I can't report any results personally from when I've tried the 90-minute interval method, but I've read on several blogs reports of people having had success with it.
toasterwater: Yeah, in most circumstances any nap under 45 minutes will not get into deep Stage 3 or 4 sleep, so while it can be refreshing in the short term, it's not enough to affect the amount of deep sleep you need at night; nor is it likely to "trap" you in a deep sleep that you feel tired waking up from. (The exception is that polyphasic sleepers, from what I've read, reach deep sleep much more quickly and visit different stages in different naps so to get the right amount of each. This explains why polyphasics can get by with just short naps.)