A lot of good info here. I'm the trekking pole editor here at BackpackingLight and I might be able to add some additional information.
First, in my opinion and experience, using trekking poles can be easier on the body and also more effecient over long mileage. I started out a naysayer but converted after using my wife's poles on some long days and through deep Utah sand. Now I use poles for every hike.
Second, there are two main styles of pole usage:
"Trekking style" mans you use the poles for stability and balance. The tips are place beside the feet as you walk and they take weight off the knees and offer additional security. These poles tend to be shorter typically fit the 90 degree rule mentioned above.
"Nordic Walking style" means that the poles are used for forward propulsion, much like a XC ski pole. Sized for this style, the poles are usually a bit longer (mine are about 5 cm longer). Here, the poles are place behind the feet and you push off for increased speed or forward momentum.
That said, you'll likely develop a blend of the styles, although you'll typically fall into one camp. For example, my wife uses trekking style 90% of the time but uses NW style when climbing steep sections. I use NW style primarily but switch to trekking for stability through really rough sections or when decending sketchy or very steep parts.
This gets you to the length question, and it's a tough one. I found my length by using adjustable poles over a period of years. I'm about 6'1" and I used to set mine at 125 for general usage and 128 (yeah, I'm anal) for snowshoeing. Now I use fixed length all the time (except when testing new models for the site) and prefer 128 to 132 cm for all times. I tend to go for my 128s for general use and use my 132 STIX for when I'm really going for it (long days, big mileage). My 128s are just a tad beyond the 90 deg and my 132s are well past that. But if I was using them for trekking, I'd be a 125.
I hope that helps a little. Ross's idea of borrowing an adjustable set to develop your ideal length is a great one. Remember to measure from the tip to the end of the handle for a true measurement. The idea of getting the adjustable Titanium Goat poles is a good one too. I've never tried those but I've used their fixed and collapsible poles- solid products. You can't go wrong with Gossamer Gear either and Komperdell Featherlites are sweet too. Stix are expensive but really stiff and light...they are the Maserati of trekking poles in my opinion (and I mean old Maserati- think 1978 Bora). :-)
You might also want to check this out: Carbon Fiber Trekking Pole Review Summary.
Last, as Ross pointed out, there is definitely debate regarding the effeciency of using trekking poles. I truly believe that I couldn't pull off the mega 40-50 mile days I sometimes do without the use of poles- I see them as a critical componenet of my system. Others such as Andy Skurka would agree. But there are certainly many folks that would offer different opinions. I'd love to offer quantitative numbers to prove my point but I only have qualitative experience. Hopefully someday I can definitely prove everyone else wrong but until then you'll have to find out for yourself. :-) Lots of ways to hike down a trail and that is for sure.
Best of luck!