How do I transport items upwards from a monster trap?
Edit: With the addition of pistons into Minecraft, YouTuber Basxt came up with a very clever way to make an "item elevator", using pistons. However, since pistons can only push 12 blocks at a time, you'd need to make many of these elevators that pass the items off to each other in order to transport items a large distance upwards.
Begin original answer:
There is no way to transfer items upwards. However, specific to your point about transporting items from a monster trap, you can instead move the mobs upwards and kill them above.
YouTuber EthosLab has a 4 part tutorial on how to make a mob trap. His system contains a tunnel that transports the mobs upwards, and the information he gives (audio) about it is very useful.
Part 1:
A horizontally "crushing" block powered by a piston would pop all items in its spot up one. You could theoretically use a "zipper" type piston shaft to pop the items upward. The only catch is that the items need to have nowhere else to go but up.
In the 1.5 update, Minecraft now includes Droppers, which can face upwards. By chaining them together with a bit of redstone you can easily move items up any vertical distance:
It's essentially just a repeating pattern like this:
