I've actually had a number of discussions about this when I taught. Kids seem to be very interested in this question. It just depends on what requirements you're putting on the term. Do coined terms count? technical words? People often seem to have an idea that there is a single, official English dictionary, when there really isn't.
The one I always here people refer to as the longest is Pneumonoultramicroscopicsilicovolcanoconiosis, abbreviated as P45, as Nare mentioned. It refers to a problem with the lungs caused by inhaling silica dust. Some would claim it doesn't count because it's technical and coined, though it has appeared in several common English dictionaries.
Methionylglutaminylarginyltyrosylglutamyl...serine, which she also mentioned is the longest technical, published word, consisting of 1909 characters. However, I'm just getting that from wikipedia, it isn't one I'm otherwise familiar with.
The chemical name for connectin is much longer, with something like near 200,000 characters(189,819 after checking wikipedia), but you probably wont find that in a dictionary and when you count chemical names you start to run into semantic arguments about how we define "word".