5:08 a.m. on December 29, 2010 (EST)
I just had to chime in here since I just came back from Machu Picchu a few weeks ago during rainy season. I did the 4-day hike on the Inca Trail and it was a totally amazing experience! The highest altitude you'll reach on the Inca Trail is the Dead Woman's Pass at 4215 meters. That's a little over 13,800 feet for us Americans. :P Day 2 of the trek is definitely the most difficult as you will be gaining the most elevation that day.
A decent backpack is of course recommended but your porters are going to be carrying the majority of the stuff for you. I went with a tour company called Intrepid which I found out was actually an Australian company. Our guide and the porters were super awesome and I couldn't recommend Intrepid enough. The porters carry your tents, food, etc. They provided us with a duffle bag which the porters would carry 6kg of our gear (i.e., change of clothes and sleeping bag). I'm almost certain that's standard practice because a friend of mine hiked the Inca Trail earlier this year with a different tour group (GAP) and they gave her a 6kg limit for her duffle bag as well.
Basically, you're going to need your backpack to carry water, snacks, rain jacket, jacket, personal affects (like prescription medication), etc.
That said, I have absolutely no recommendation for you as far as backpacks go. Hahaha. I don't know enough about them to offer you any good advice. Sorry. :P Most of the hikes I do are about 6 hours long so my Camelbak is good enough for that. :P I actually did use my Camelbak for the Inca Trail but I left the 3L bladder at home and instead brought two Nalgene bottles with me. The porters will provide boiled water for you and I didn't think it'd be a good idea to put boiled water in my hydration bladder. In retrospect, the water was never piping hot when they gave it to us, it was always cooled down to room temperature by the time they gave it to us so taking my bladder with me would've been okay.
As mentioned above, I assume you'll be in Cusco at some point prior to making the trek so that will give you some time to acclimate to the high altitude. I live in Hawaii, which is of course at sea level so I wasn't used to the altitude when I got to Cusco. Lima is at sea level, so hanging around Lima isn't going to do you any good as far as acclimating to high altitude. I run and hike a lot in Hawaii but there was no way for me to "train" for the altitude. That said, if you're in decent shape cardio-wise, you'll be fine. The main thing is to remember to take really deep breaths. :)
All the shirts I brought with me on the hike were Patagonia shirts. 2 Capilene 1 shirts (one short sleeve, one long sleeve), 1 Capilene 2 shirt (short sleeve), and 1 Capilene 3 shirt (long sleeve). I also brought a Patagonia Rain Shadow jacket which came in quite handy as it rained quite a bit during the hike and a The North Face microfleece for when it got cold (which is relative...again, I live in Hawaii). Of course there are no shower facilities on the trail, so depending on your personal tastes, you may not even need to change clothes...though you should probably at least change your socks and undergarments.
Anyway, I could go on and on about the trail and what I brought, et cetera, et cetera, but I've already written way more than what I thought I was going to write. You've probably already started your trip anyway, so you might not even read this in time! Haha.
Oh, one last thing. You *can* use metal hiking poles on the trail. In fact, our guide said that was recommended instead of buying wooden poles from a vendor in one of the small towns near the start of the trail. I can't remember for sure off hand, but I think he said it was better to use the metal ones for ecological reasons....which is ironic, I guess. I guess people are cutting down too many trees and what not. Once you get INTO Machu Picchu, however, you're not *supposed* to use hiking poles because they don't want the poles ruining the ruins. For people who obviously need poles because of physical disability or whatever, though, they will make an exception.