I can't really remember where I read that, but something comparable was written in The Savage Garden:
''Some species of pings also have the power of movement. This is most often seen in species from temperate climates; it is almost entirely lacking in tropical forms. Over a period of a day or so, after the capture of substantial-size prey, the margins of the leaf, already upturned, may curve inward or over the precious food. This has nothing to do with capture but is believed to be helpful in preventing the digestive fluids from drooling off the leaf. Another possible explanation is that it helps prevent the victims from being washed away by the rain. Many butterworts can even ''dish'' their leaves under prey, giving their juices a convenient place to pool.''
http://www.sarracenia.com/faq/faq5440.html gives another explanation though:
''For the most part, there is no motion in the leaves. However, the leaves do often dimple slightly underneath captured prey, possibly to create a little pool of fluid to aid in digestion. Also, and especially on temperate species, the leaves roll up on the edges. A few theories have been proposed to explain this, and perhaps two of the most intriguing are that the leaves may be curling up to keep marauding ants from stealing the captured prey, or to create a kind of tubelike structure along the edges of the leaves so that capillary action spreads the nutrient-rich bug juices over a larger amount of leaf area, enhancing nutrient absorption.''
I guess science still isn't sure why they do this