My item level is up to 348, but I am the first to admit that value is somewhat spurious. Item level takes the highest level item that you still possess that you have equipped in a slot at some point. As a result, you get credit towards both finger or trinket slots based on your highest level ring or trinket, even if you only have one of them and thus couldn't possibly use it in both slots.
In fact, my armory says that my item level is 348 but conveniently notes in brackets that I have item level 337 equipped. Why the big difference? It's Sliver of Pure Ice from Icecrown Citadel.1
It's not that I've been extremely unlucky with drops and have never acquired a trinket from a dungeon. But I've never acquired a better trinket from a dungeon.
Case in point: I have a Blood of Isiset in my inventory. It's 69 levels higher, so it seems like it should be clearly better.
Blood has a 105 second internal cooldown and a 10% proc chance. Right now I'm casting a spell about every 2.5 seconds, so it should average around 25 seconds to proc again, giving it an overall cooldown of 130 seconds. So it gives 1512 spirit 15.4% of the time. That's equivalent to 232.6 spirit all the time. Spirit provides 0.1225 mana per second with my current intellect, so this is about 28.57 mana per second overall. Intellect provides 0.0733 mana per second with my current levels of intellect and spirit, so the 152 intellect is worth 11.13 mana per second. The use ability on the Sliver is pretty easy to work out, 1830 every 120 seconds is 15.25 mana per second. Looks like blood wins on mana by 2.49 mana per second, the equivalent of about 20 spirit.
Blood provides 252 mastery instead of 152 intellect. I haven't parsed any of my logs yet to get a good casting pattern, but we can easily compare these things for two of my most frequently cast spells: Greater Heal and Prayer of Healing. For Greater Heal, Intellect is around 6.5 times as good as mastery for increasing throughput. Furthermore, Intellect is consistent and mastery is random as it is dependent on criticals. For Prayer of Healing, aegis triggers every time, so Intellect is only 2.9 times as good as mastery. In the best case scenario, then, the Sliver is around 75% better in terms of throughput. If we converted the throughput from Intellect into master for a direct comparison, the Sliver would be around 189 master rating better. Bear in mind that this is being extremely generous towards mastery rating by assuming that I cast virtually nothing but the spell where is shines the most.
I don't want to directly compare throughput to mana, but if you asked me whether I wanted 189 mastery rating or 20 spirit, I think the choice is fairly clear.
The developers recently mentioned that they know that "secondary stats" are worse than primary ones. So spirit, mastery, crit, and haste are worse than Intellect. I think they may be a bit delusional, however, about exactly how much worse they are. This is spec dependent - I think Blood would clearly win out over sliver for a Holy priest - but 69 items levels for a trinket itemized for the same spec should guarantee a stronger item. Instead, the most important thing to look for on a trinket is whether it gives a passive bonus to Intellect. Item level comes second.
1. It's also because I have my Runed Signet of the Kirin Tor equipped from a recent teleport to Dalaran.