as far as i've seen, the ds isn't all that hard to take apart. you will, however, need nintendo's proprietary tri-wing screwdriver, else you'll have a hell of a time getting into it.
it's probably just as you say the spring's mis-aligned, not causing the button to spring back up right, or that the spring's snapped for whatever reason.
if you manage to get hold of one of these screwdrivers, and decide to take apart your ds to attempt to fix it, be very careful of the lcd connectors, and cables. if these become misaligned, disconnected or damaged in any way, you can say bye bye to your ds, and probably another 100 bucks to buy a new one. not that i'm trying to scare you off doing it or anything, just a forewarning to be careful of the lcd's connections. granted you probably won't need to take the lcd's off the chassis to get to the shoulder buttons, but it's also a just-incase.
also, there's about 3 screws hidden behind the battery compartment. i can't remember if it was the ds they were talking about, but there's something like 25 screws holding that thing together. may have been the wii though. not sure.
good luck if you decide to try it though =)

here's something which may help you disassemble it...
http://uk.gizmodo.com/2006/11/03/nintendo_ds_lite_totally_disas.html

it seems, to get to the shoulder buttons (and the mechanism), you only have to take off the back cover, which has nothing attached to it. the motherboard is screwed to the side that the lcd's on (the front, basically).

lol @ that article. overclocking the ds. one question that never got answered there though, and the one i really wanna know the answer to, if you overclock your ds to 1.8x, what happens in multiplayer mode? lmao

as for the springs. test the buttons themselves (use a small flatblade screwdriver and see if they're springy enough as well). failing that, when re-assembling, just bend the wings on the springs in (or out) slightly more, so when you reposition the wings, it should have a slight more of a "push" on the button, if you know what i mean.

probably helps with the rendering on opera. i'm not sure why you'd use the ds to browse the internet though, much like why you'd use the wii to do the same thing, since there's a perfectly good pc in the house that can do a better job on a bigger, finer resolution screen.
that aside though, i am gonna get the opera browser on my wii, and attempt to get cuccoscratch working correctly on it. it'll just be a gimick more than anything else, but at least we can then say our forum skin is wii compatible lol