I could see that it might be possible for them to pull the rug out from under us and someone else have shot Shannon, but this source's story makes no sense.
As the person above me mentioned, Ana Lucia certainly thinks she shot Shannon, and it sounds like the "source" hasn't seen the newest episode yet. Plus, Shannon's wound was definately not the result of being stabbed by a sharp metal object, it was too neat and bullet-like. The only possibility is if the other's (or Danielle) shot her and Ana Lucia's shot went somewhere else in all the confusion, but in the last two eps it looks like you can sort of see Ana actually shooting her in the gut. I woudln't be surprised if Lost found someway to reveal someone else shot her, cause it something they would do, and yet from what we've seen so far it really seems like Ana has to be the one.