This is simply a situation where, IMO, the game would be improved by adding 2 sentences to a 448 pg book to avoid conflicts between the 'common sense' between parties at the table, as demonstrated within this thread by the quotes from Sturn & Kallabecca, below.
Unsurprisingly, I disagree.
Maybe I've been playing a few too many indy and indy-flavor games of late, games that put the power back into the GM's hands rather than forceing a plethora of rules on both player and GM, granting the freedom to apply "common sense" is a better approach than having a spelled-out rule telling you "well, your common sense should tell you this."
That said, perhaps those two items could have a slightly expanded explanation (if it's not too late to add such a thing), that "backpacks" and "utility belts" are just loose descriptors, and that said items can be used to represent various other means of storing/carrying gear, much like the handful of examples I gave above, within reason.
Someone carrying three "utility belts" (an ammo bandolier across the chest and pair of Old West style gun belts with holsters) isn't that improbable, and personally I think it's a bit on the cool side, particularly if the character is a gunslinger packing twin blaster pistols. Mechanically, it'd just be simpler to buy a backpack (cheaper too) and be done with it, but I at least think it's a cool visual, and the GM should work with the player to accomodate that concept.
I think Doc, the Weasel has the right idea, and that a GM should be able to allow simple "common sense" inform their decision of what is and isn't kosher without needing to be told as much. Someone trying to wear four backpacks (as in actual backpacks) is absurd, and I'd be surprised if a GM didn't put the kibosh on something like that. But an actual backpack and a belt-rig for a tool kit (both "backpacks" in game terms) is a good deal more reasonable.
Then again, maybe I'm placing more faith in the many GMs out there than others might, but I figure the vast majority of GMs really don't need as much hand-holding rules-wise as D&D 3rd Edition generally assumed was needed (granted, it wasn't as bad as Rolemaster got at points, since I never saw a chart for blowing your nose in an official WotC product). Maybe it was the result of hearing too many horror stories of power-mad Dictator!DMs and allegedly "spineless" DMs that couldn't keep their players in check from 1st and 2nd edition that caused WotC to take the approach they did with 3rd edition and trying to codify every little thing, and if anything only serving to re-inforce the "DM vs. Players" mentality that's been the bane of many a promsing campaign. But that's not really the point of this topic and kind of a moot point anyway since nobody here was involved in 3rd edition's development.