Here is my experienec so far with my ER4-Ps and getting some tinnitus.
When I first got the Etys, I used the flanged tips and, due to the size of my ear canals, they went in very deep, including some of the black tube of the actual earbud. The result, due to me having some earwax already in my ear (but not causing any problems) was that the flanged tips compressed the wax against my ear drums. I then started getting muffled hearing, a ringing in my head and I also got a slight infection in the left ear. All this was coupled with getting a bad cold at the same time.
I have since had my ears syringed and the ringing is still there. I have seen an ENT surgeon and he did many tests. Both ears are clear of wax and any infection, and my right ear is fine in all tests. My left ear has a drop off at very high frequencies, but I am not sure if I couldn't hear them due to hearing the ringing instead. Both ears are asessed as being within the limits for a man of may age (45).
The upshot is that the ENT surgeon reckons the tinnitus will go and that it is not caused by any exposure to excessive sound but more to the irritation to the inner ear caused by the wax, cold etc.
So why am am writing this? Well, two pieces of advice for anyone contemplating using in canal phones.
1. Get your ears syringed before inserting anything, because you can get tinnitus from aggravating the ear drum through compressing wax against it. Remember, tinnitus is a condition and there are many causes and types. I am hoping mine is temporary from the combination of all that happened to me, but the wax compression definitely started it off.
2. Use very low volumes when using Etys or similar. You will find that with a good seal, and after the first few minutes, the volume will be very audible. On an iPod, 25-33% is all you need. I burned my Etys in and even at this level I can get excellent sound with defined bass.
Anyway, hope this helps some of you avoid any problems. In canal phones are the nearest you will get to plugging your head into the sound, and they are worth the effort to use properly and sensibly.