Note: Edits are in red
Of course, it wouldn't be as easy as simply placing a saddle on a spider. You would actually have to tame the spider. Here's how it would work:
1) Find a spider (preferrably during the day) and make it notice you
2) Drop food for the spider to eat. Depending on the food, the spider will be tamed faster (EDIT: Both mushrooms can be found aboveground, added fish):
- [*:34pr3d44]Raw Fish: 12 fish
[*:34pr3d44]Cooked fish: 10 fish
[*:34pr3d44]Raw Pork: 8 slabs
[*:34pr3d44]Cooked Pork: 6 slabs
3) Once the spider is tamed, its eyes will turn from red to green, and will not attack you at night
4) Now you have a tamed spider!
The spider mount (you have to place a saddle on him) can jump 2 spaces high, and can travel twice as fast as you. I also thought that when you attacked with a weapon, the spider also attacks, giving you more damage per attack.
I also figured that these spiders, aside from riding, could be used as guards. You could craft web tile, an item that is now in the game files:
  
(where = String)
And place them like pressure pads. The spiders would walk along any adjacent web tiles you place, so you could make a "patrol area" (guard spiders FTW). If a mob crosses a web tile, the spider on that tile will begin attacking that mob. The spiders could take damage, but you can heal spiders by feeding them.
The spiders would stay friendly until you attacked them (or possibly if you don't feed them for 1 day), but they will wander away if not corralled by web tile or a fence.
The addition of a horde of spiders guarding your place would make for one badass fortress
HOW THEY'RE DIFFERENT FROM WOLVES
- [*:34pr3d44]Spiders take more effort to tame, which is balanced by the fact that they're more useful individually.
[*:34pr3d44]Spiders don't directly follow you when tamed. Instead, they'll follow the nearest web tile or wander around like normal.
[*:34pr3d44]Tamed spiders won't be directed by mob attacks. Instead, they act like they normally do, except against intruding mobs rather than you.
[*:34pr3d44]They'll attack creepers
[*:34pr3d44]You can ride them.
[*:34pr3d44]You can;t make them "stay" like wolves. Instead, a tamed spider will keep its location even outside of a chunk if there is nearby web tile. Same principle, different method.
Yes, it would work like walking, but you're bulkier. When you're riding a spider, you would take up a
Yes, you could walk up walls with spiders now. Imagine making a PvP game where everyone rides spiders-- or a spider-rider vs castle defenders game!
Here's a link to the GetSat topic I'm posting on. Get it noticed!