This reminds me of a similar post from a few weeks ago (only the body part of choice was ears, lol):
http://www.mamapedia.com/questions/17641698000038395905
Like I said to the other mama, I think it's VERY normal and most likely subconsciously associated with the comfort of nursing. The only difference here is that she thinks she NEEDS it to sleep, which has turned something of comfort into a bit of an addiction. So my advice is to handle it just like you would if it WERE a pacifier or a bottle or anything else toddlers get "hooked" on.
At two and a half, she can be somewhat reasoned with, so you might want to try a compromise. Are you still rocking her to sleep at night? If so, that might actually be what she's addicted to...not just the elbow. And if she's addicted to the rocking....well, I can think of worse ways to spend a few minutes every evening, but it's important that you put her into her own bed before she is completely asleep.
If you're not rocking her and are literally stuck leaning over her bed at night while she strokes your elbow, then clearly something needs to be done. You might consider going BACK to rocking her at night so she gets that special comfort time and laying her down while she's still awake (but just barely). If she fusses, I think she's old enough for you to explain that your elbow is for when you're rocking her and that's done for tonight. Be consistent and it will stick eventually. Over time, you can shorten the rocking time by small increments and eventually quit altogether if you wish.