I dropped off my 2005 Pathfinder at the dealership yesterday afternoon to get an oil change and to look into the squealing from the auto-tensioner. From what I've been able to find on the internet (clubfrontier.org, frontierboard.com, nissanpath.com, and nissanhelp.com) is that there are two separate TSBs for similar noises:
NTB06-072a is for "Drive Belt Noise" that involves the drive belt and auto-tensioner. These parts are apparently not covered under the powertrain warranty, but are covered by your vehicle's 3 year/36,000 warranty.
NTB07-042a is for "Buzzing/Whining Noise From Timing Chain Area" that involves secondary timing chains and both timing chain tensioners. It looks like service related to this TSB should be covered under Nissan's 5 year/50,000 powertrain warranty (according to my 2005 Nissan Pathfinder warrantly booklet, "timing chains and tensioners" are covered).
Based on what others have reported and what the dealership mentioned to me over the phone, replacing the auto-tensioner and drive belt costs approximately $250 to $300. Based on what I've read online, repair costs associated with the latter TSB is approximately $2,500.
Luckily the noise I've been hearing is due to my auto-tensioner and not the timing chains.