There's an article on Gnome Stew about the horrors of a player's equipment getting destroyed. Of course, their analysis and solutions revolve around new school concerns (it's OK to destroy common, easily replaced items willy-nilly, but destroying Excalibur might de-protagonize a player or ruin your story!) I have other concerns. But I thought I'd write a bit on the topic anyways.
Concern #1: How to determine when items are destroyed. Simple in my book: d6 roll, 5+ means items may be destroyed. Which items and when depends on the destructive force involved and the conditions present. For example, a fireball will primarily endanger flammable items, starting with the outermost items and burning their way inwards. So I wouldn't roll a die and say "your scrolls are destroyed!" Instead, I'd say "you catch fire!" Then roll another d6 for the number of rounds before things inside packs and pouches start to burn. If the player reacts fast enough, the scrolls may be safe.
Concern #2: Magic items. These (and their big brothers, the artifacts and relics) are the only items I would even consider some kind of immunity for. Not because players get attached to them or because the story demands it, but because magic items are magic and are supposed to be supernormal. First, let's be clear that we aren't talking about one-shot or consumable items, like potions and scrolls and probably wands; as I suggested in Concern #1, these get treated the same as everything else. Second, if an item grants some kind of protection (fire resistance, for example,) it should probably be invulnerable to that specific destructive force.
Within those criteria, my general principal is that magic should be countered by magic. A spell that starts a mundane fire won't melt Excalibur, but a truly magical fire could, perhaps. That does open the question of whether a fireball is normal or supernormal; I think the safest route is to make the answer depend on caster level. Permanent enchantments are the creation of wizards (11th level,) so either a spell with damage based on level, cast by a wizard, or a 6th level spell or better, is necessary for there to be a chance of destruction. Furthermore, if two magic items are pitted against each other, their magic ratings should be compared. For things like magic swords, the magic rating is the plus rating; other magic items are equivalent to +1 swords unless defined otherwise. Pitting two items of equal power against each other may harm either or both. Pitting a +2 sword against a +1 sword would only endanger the +1 sword, though. Artifacts trump any magic item, and conflicts between two artifacts must be determined on a case by case basis (but usually, artifacts can't be destroyed except by specific circumstances.)
If a magic item is endangered, there's a separate d6 roll for each item. If the magic endangering the item is more powerful, there's a +1 to the roll; if a +2 or better item is struck by an 11th level spell effect, there's a +1 to the targent number (in other words, it is only harmed on a 6 instead of a 5+.) I'm debating whether half-strength magic (level 6 through 10 caster) should still have a chance to harm the item, at -1 to the roll.
All this is a big change from some of my earlier thoughts on the subject, which involved a general save for all the items and individual saves for items the player wanted to protect. In this case, most of the time there's not as many dice to roll.