The folks building Northshore in Canada have one huge advantage, which is access to a pretty much an unlimited supply of fallen cedar which doesn't decompose terribly quickly.
The manky pine that many pallets here are constructed from is thin timber and it's also pretty low grade. In the outdoors environment it doesn't last terribly long.
Some pallets are made of longer lasting hardwood so they may survive better but the planks are still pretty light weight if you plan on constructing any kind of long term structure.
Please, if you're going to build timber structures, do it either on your own property or somewhere where the land manager has given permission. Half arsed construction standards and illegal building make MTB'ers look like an environmental problem rather than a legitimate user group and make gaining access to areas to ride even more difficult than it presently is.
the blue pallets are hardwood, but for anything that touches the ground, use treated pie if you can, otherwise soak the wood in old engine oil/hydraulic oil works pretty good.
for the part you ride on as long as it doesnt touch the ground try to use hardwood, but normal pallet flat planks are fine as long as you check them every so ofter (once every two weeks should do it)
like the others say, try to follow the format of the existing stickies, people are more likely to respect what you say, and give positive feedback or constructive criticism.
like FR Drew says, make sure you have permission to build on the land you want to use, or you will have police, angry residents, and eventually other MTBers it affected to answer to.
Good Luck with future farkining.