@ Angus Wood
Naked shorting does affect the market differently to borrowed shorting. By naked shorting, you artifically increase the supply of the stock, driving the stock down. The listed pricing of the stock is the last executed price, not the mid-point between the closest ask and the bid prices. If the price is articifially depressed, and someone is leveraged in the stock targeted, the drop in price can cause a margin call and trigger a sell-off, further driving the price down.
And "x" and "y" do have a relation. Unless it's an illiquid stock, or there will be some price sensitive news breaking soon, the bid/ask prices are normally quite close together. If there's a huge spread between the prices, there are always people willing to sell/buy at a better price than you. Naked shorting tells all the buyers that they can get their stock for the price they're asking. This in turn means that the sellers have to offer lower prices or get undercut by someone else offering the stock between the new bidding price and their asking, which drives the price down again, etc etc.
With covered shorts, the seller has to, at some point, buy the stock back. The seller also has to find, and borrow (read: pay a fee for), the stock. If too much is being borrowed and shorted, the lenders are less likely to lend out more and risk the value of the stock they're still actually holding. (At the end of the day, they know they have to take their stock back. At some point.)
So, having driven the price down, transactions are now being completed at the lower price, and the naked shorter can now cash in all his options and merrily go on his way.
The equivalent opposite, a "naked long" I guess, would be to promise to buy stock when you don't have the money. And that just sounds silly from the get-go. And before anyone says "that's stupid", a covered long would then be taking a loan to buy some stocks, with the plan to make some money and pay the cash back + interest. Which is what people do.
... ... ...
I'm sure I had a point in there somewhere...
Ah, yes. Excessive naked shorting can then be construed as market manipulation, which I believe is illegal.
Death, in memory of Overstock and Bear Stearns...