Well, if you're sticking to the D&D concept, I'd definitely make this a Paragon Path available to drow clerics. Its features and powers would involve a climb speed and some web-slinging and poison bites and maybe some fun "Lolth is such a psycho" powers.
If you are just looking people with spider-bodies, look at the existing race's class features and such, and just make it up. They almost certainly have a climb speed, which I think would be worth a LOT, so they'd probably have very few other useful traits. Their encounter power could maybe be webbing-related: a ranged standard action to immobilize (save ends). And they can produce silk ropes whenever they want.