For some types of DSL, yes, 2.5 miles (about 13,000 "wire feet") is "pushing it" but for modern DSL, it isn't completely outside the range of what is possible, especially with new wiring plant (as opposed to trying to clean up some 40 to 50 year old phone installation of that length for DSL service).

About optical runs: often, trying to "tap into" an optical service takes big bucks unless the infrastructure is already in place in that hub. Sometimes, optical service that "runs by" your house, town or business is more than a little like a 230kV long-haul power transmission line that runs "right past your ranch... that is off the grid." Sure, there is electricity so close that you can see it out your front door -- trouble is, a substation to take that high voltage (or high speed) long-haul link down to your local needs is very, very expensive. So if they come back with statements to the effect of "sure, we'll give you a tap into the optical fiber for... $50K (or higher)" -- it isn't necessarily BS. While at cisco, I never gave a second thought to how much it cost to tap into a OC-12 or OC-48 link - because we made the equipment. Every now and again, a project manager would come by and inquire "Say, where did you put that OC-48 line card... can we have it back, please?" and then he'd let me know "Well, you know how it is when you loan out a line card that costs $50K... we kinda like to keep track of it..."

Now that I help folks figure out what they need... well, let's just say that to tap the fattest pipes takes the fattest wallets. Once you start trying to tap into a circuit that is faster than a "DS-3" (45.foo Mbps), your prices start climbing very, very quickly. Most long-haul fiber links are now at least OC-12's.

Here's a handy decoder ring to this "OC-nn" gibberish you keep seeing me use:

http://en.wikipedia.org/wiki/Optical_Carrier

Where you are, I'd guess (SWAG) that a fiber link is OC-12, maybe OC-48.

Here's more info on the "DS-n" stuff I keep tossing around:

http://en.wikipedia.org/wiki/T-carrier

For 5.8Gig WISP equipment, 7 miles isn't "too far" -- especially if you go to more directional antennas. For the backhaul (point-to-point) equipment, 7 miles is child's play -- some of these 5.8Gig units made for backhaul do 40 miles easily. So for setting up a WISP, if the telco doesn't have any service suitable in those lines that run near to your place, then I'd think about finding someone in Burns who could co-operate and give you some space, power and access to the telco network there in Burns, where you likely could get some data service, then backhaul the bandwidth out to your tower.

BTW -- DSL isn't the only wired option. There are plenty of other data options, many not often seen by consumers. For setting up a WISP in your area, I'd also be inquiring as to the availability of a Frame Relay link to the Internet at the point where your wireless traffic wants to go onto the wired 'net. Our bonded DS-1's are Frame Relay links back to Reno. I'm almost absolutely certain that you could find a FR switched service closer than 250 miles away from you...

Here's info on Frame Relay:

http://en.wikipedia.org/wiki/Frame_Relay 

The reason why Frame Relay is viable is that the physical infrastructure is the same as a "leased line", aka "T1" -- but the telco gets to tariff the service differently than a leased line. FR links are not all that expensive if you have enough users to pay for the monthly fee. Fees are typically tariffed in part based on the distance, partly on the speed, and partly on how much 'committed information rate' your provider gives you on the link. With FR, your provider can say "OK, we'll give you 1.544 Mbps when we have enough bandwidth (which will be the vast majority of the time), but when our network gets a little clogged up, we'll commit to only 256Kbps (or something similar)." This cuts the price of your wired data service substantially down from getting a full, 100% of the time 1.544Mbps bandwidth commitment from the carrier.