Silverfish live inside some of the blocks in strongholds and extreme hills (plus a spawner at the foot of stronghold end portals), and pop out when you break the block. They're not really that dangerous - more annoying than anything.
The endermite is new and used to look just like a silverfish, but purple. Since yesterday they now look like
this. Apparently they sometimes spawn when an enderman teleports or if you use an enderpearl to teleport. They're a bit more dangerous than the silverfish (they deal 2 hearts damage; the endermite deals 1). I doubt they'll affect the End Trap, as the endermen can't teleport out of there, but we'll have to wait and see.
Other than the mite I haven't heard of any new mobs planned for 1.8, but you never know.