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There's actually a few small intermediary steps, to my understanding, in Rychan's otherwise neat explanation:
- Plants with more color and/or small pockets of mass that will catch rain water and accumulate sugary liquid from the plants glands or pollent are selected because they attract more insects for pollination. (Significant because it describes the creation of what will become a trapping ground later.
- Plants are selected for larger and larger liquid-holding pockets, which eventually evolve into phytotelmata.
- Insects fall into the phytotelmata, and local bacteria and parasites digest the insects, leaving basic nutrients that the plant can then absorb. (This bridges the evolutionary gap of the plant evolving the ability to trap insects at the same time as evolving the ability to digest the insect, a la blind watchmaker.)
- The plant develops small mutations (downwards-growing hairs, slippery sides) that lead to insects becoming trapped at a greater percentage. Some pitcher plants' evolutionary journeys end here.
- The intercellular methods of absorption of the digested insect nutrients are developed and eventually the plant evolves the ability to break the dead insect down into its basic amino acids through the production of proteases and phosphotases. (I haven't come across one definite mechanism for the evolution of the ability to create these enzymes, although viral transduction/transmission seems to me to be a pretty good possibility.)
- The modern day pitcher plant is born.

Carnivorous plants are a fascinating topic of biomechanics and evolutionary mechanisms. Some neat papers and links to check out:
http://www.botany.org/Carnivorous_Plants/
http://www.skepticfiles.org/evolut/meatplnt.htm
http://www.botanical.com/botanical/mgmh/p/pitche42.html

Cameron, K.M., Wurdack, K.J., and Jobson, R.W. 2002. Molecular evidence for the common origin of snap-traps among carnivorous plants. American Journal of Botany 89:1503--1509.
Evolution of the Genetic Architecture Underlying Fitness in the Pitcher- Plant Mosquito, Wyeomyia smithii
Peter Armbruster, William E. Bradshaw, Christina M. Holzapfel
Evolution, Vol. 51, No. 2 (Apr., 1997), pp. 451-458
http://links.jstor.org/sici?sici=0014-3820(197506)29%3A2%3C296%3AEAEOTP%3E2.0.CO%3B2-9