Yes, I got my mining up to 375 yesterday – woot! The Fel Iron can be mined all the way to 375, it’s still green. For some reason I was unable to get the last two points by mining Fel Iron, but luckily I found a couple of Adamantite nodes that pushed me over the top. Adamantite is the next level of ore after Fel Iron. I haven’t actually used this stuff in any recipes yet, since my Smithing is still at 330 and the Adamantite recipes appear to kick in around 335.
The last type of ore you can mine, which requires 375 mining, is Khorium. This seems to be a rare ore (the name is green, not white), although I’ve seen a few nodes of it in Hellfire Peninsula and Zangarmarsh. So I can’t say if it’s rare everywhere or just rare in the two noobish zones I’ve been frequenting. There’s also a rare ore called Eternium, which I’ve found even less of. It seems to sometimes spawn as it’s own node and sometimes drop as a bonus ore from a regular node. I got two early on as bonus drops and a few days ago saw a node of Eternium itself that I mined.
I’m really not sure what level is required for Adamantite or Eternium, by the time I found them I was able to mine them. I’m guessing that Adamantite kicks in around the mid 300 (since that’s when the recipes start) and I have no clue on Eternium since it’s a rare.