I've come to see LEW as a pretty open place. If we have PCs running about who are orcs, goblins or kobolts, why not Drow? At this point, I don't know that there are any active 'monster race' PCs, but it seems within the rules. Given that we allow subraces for other races, such as elves and halflings, which aren't necessarily as well balanced as the standard races, as well as monster races, I don't see why a half-drow shouldn't be used. Parties from persistant worlds such as this are rarely composed of characters of all the same level. The concept of inter-PC 'balance' isn't as much of an issue.
Drow are so interesting to so many people, I'd like to see this approved. The half-drow as written seems fine to me. No favored class is listed. I assume this means it has no favored class (like a human or normal half-elf). The drow in this proposal is more troubling. As has been stated, Drow are in the SRD and are essentially 'pre-approved'. Correct me if I'm wrong, but the drow listed in the proposal are identical, except for the noble blood thing, which I'm not too keen on.
Something you might consider, Spycat, is making several versions of the half drow. The obvious question being, what is the other half? I assume Drow can breed with other elves, and perhaps other underground creatures. While for surface elves, humans seem the most likely non-elvish mate, they don't seem particularly likely mates for Drow. Identifying the other half (or at least defining it for this proposal if you don't want to make multiple versions) might make the half drow more understandable.