This is just a quick thought I had last night while reading HBP, so bear with me.
We know nothing of Snape's parents except his mother is Eileen Prince, his father was a muggle, and by the looks of it Eileen Prince was at school at the same time as Voldy. We don't know if 'defying' LV means defeating him at something, but it is possible that Eileen Prince could've got one over... or possibly three over... Tom Riddle at school... this could mean 'defying' the Dark Lord too, if you are an exceptionally cheesy thinker.
We don't know Snape's birthday, do we? However ironic that it is the same as Harry and Neville, possibly Snape's two most hated students, it is a possibility that it is near them as well.
And he has power the Dark Lord knows not... what I was thinking is this could mean two things. If Snape is double-crossing LV, he would have to use Occulemency (sp) against LV. Now, if LV knew Snape was having to use Occ. against him, he wouldn't be very happy, as he would know Snape was up to something. But since Snape is still with LV, the Dark Lord knows not of this power.
Secondly, it could still mean love. I read somewhere else that the Unbreakable Vow at the beginning of HBP is very similar to wedding vows. It is possible that Snape and Narcissa love, and therefore have power that the Dark Lord knows not.
And to think in OotP, LV picked out Harry instead of Neville, so the prophecy is definately about him. Now, most will think that because the prophecy said 'Harry Potter(?)' it is a reference to Neville as well, but this could be telling the reader more than it wants. This could mean that it still may not be Harry.
This may be utterly pointless and someone may bring up some evidence that means this last 10 minutes of typing was wasted, but I just wanted to point that out.