Josh,
It's strange being on the other side of the world to you guys. I go to bed and wake up and lo! There has been lots of activity on the forums
There will be many people around on this forum that know more than I about the technicalities of trigger mechanisms for flowering in Nepenthes, I can only relate what I have observed, not what actually makes it happen from a technical viewpoint.
The age at which a Nepenthes can flower seems to vary dramatically from species to species. For example, N. ampullaria seems not to flower until it is realy quite mature, as with other swamp dwellers such as N. bicalcarata. This may be to do with mechanism for seed dispersal in relatively windless habitat, the plant has to be quite tall to disperse seeds efficiently. However by contrast, N. campanulata will flower at less than 1 year out of tissue culture and keep on flowering. This species grows excusively on limestone cliffs, so the seeds would probably not need to travel too far, therefore small plants are just as efficient at seed dispersal as larger ones.
As to trigger mechanisms, if a plant becomes suddenly stressed and is mature enough and has sufficient reserves, it will likely flower. Some orchid growers put their best orchids outside the greenhouse for a night in the frost to promote simultaneous flowering for a show a month or so ahead.
With Nepenthes a suden increase in light levels is certainly a trigger. We are now planting Nepenthes outdoors all over our land and yes, the leaves do burn in direct sunlight but the plants also flower immediately if they are mature.
I'm not entirely sure about drop in humidity or change to other envronmental factors but they probably will promote flowering if the change is sudden enough. It's a survival mechanism, "Oh I'm going to die! Must reproduce!" It's probable that a sudden change (for example increase in light level) can be made for only a day or so before restoring normal conditions and flowering may result. This is probably safer than making the change permanent which could kill the plant.
Nepenthes adapt to gradual changes in their environment very well and won't necessarily flower as a result. Sudden change seems to be the key and the change has to be in a particular direction. For example, lowering the light levels dramatically will stress the plant for sure but is unlikely to flower as a result.
I do hope Christian Klein won't mind me mentioning his collection specifically but it's a good example of a collection with a very high proportion of flowerng plants. Chris grows his Nepenthes in greenhouses with little or no shade and natural light in the summer. Summers in Germany (like elsewhere in Europe) can be overcast for long periods then followed by periods of bright weather. Chris' conditions are harsh at that time, with the result that his plants are hard and they flower all over the place. I believe that the year I visited him he had counted over 120 flowers that summer.
Having said all that, sudden environmental change may result in a terrible looking plant or even losing your plant without the desired flowering, so do be careful and try it with plants that are not too valuable first! If you want your plants to look flawless, nurture them but don't expect many flowers.
Final example: Earlier this year I visitied a botanic garden that had recently had a staffing shortage to it's CP collection which had resulted in the Nepenthes being neglected. Compared to a year previously, the plants looked generally terrible and many appeared to be suffering from a fungal disease evident underneath the leaves. Dead and dying plants were still amongst the relatively healthy plants (presumably for administrative reasons, plants cannot be simply thrown out quickly as they would in a commercial nursery or private collection). The notable thing is that nearly moribund plants were in flower. The previous year I had visited when the plants were in a lot better shape and don't recall seeing any flowers at all. So, near-death experiences for the plants may be good if all you want is flowers!
Better stop here I guess before this turns into a book. Sorry, do get carried away sometimes!