My glovebox latch spontaneously failed at the beginning of a three-day trip
(luckily I've moved my registration/insurance papers out of there, so I was
only locked out from my gum..), and upon researching the problem found that
most people hack-saw the latch off, while a few others drilled through the
door itself to access the mechanism. Neither option appealed to me as I'm
a cheapskate, and didn't want to have to buy a new door or latch.

I had access to a neighbour's 850, so I could practice on his (non-stuck)
glove-box, and the latch-mechanism is actually rather easy to pick. I was
able to fashion a suitable pick out of a piece of large-gauge aluminum wire
(copper would probably be even better). I hammered the last inch of the wire
flat on an anvil until it was about 1/4" wide, and then bent the last 3" of
the wire into a "C" shape like so:

--------------------\
\
|
|
/
/

(the flattened part being the end, of course). You then have to locate the
precise location of the center of the latch, and work the wire sideways
in between the door and the dash at that location. Carefully rotate the
wire until the end is sticking into the rectangular hole were the door-side
of the latching tooth is, and if you have all the dimensions right (length,
curvature, etc), a little wiggling in and out will spring the latch.

As I said, practicing on a non-stuck glovebox makes this much easier. People
say that the hack-saw technique takes 30min to an hour. This took me about
15min, and when I was done, no damage or parts to replace. My failure was
the usual one, with the little pot-metal pin having been sheared off, and I
drilled/tapped a hole there and replaced the pin with a set-screw, so now
my glovebox should be good for the life of the car. And no money to the
Volvo stealership!

Hope that helps someone!