Ok, here are the FACTS...
Credit card, debit card (with VISA or MC logo), cash, travelers checks, Disney Dollars -- you can use any of these to create a room charge.
WDW sets upper limits for room charges based upon level of resort -- $500 for Values, $1K for Moderates, $1.5K for Deluxes. Like I said, these are UPPER limits. You can request a smaller amount.
NO HOLD is placed on credit or debit cards. They will run the card to make sure it is from an active account, but NO HOLD is placed.
Your personal financial info is NOT on the room key. It just tells Disney to charge the purchase to your room account; your credit card number isn't encoded on the room key. The name of your financial institution isn't even on your room key. If you lose your key, no one has access to your financial information, and that's the same whether you're from Manhattan, NY or Manhattan, KS.
There is no cumulative limit on room charges. When you reach your limit, you can just set up another one, it's that simple. Let's say you set up a $500 room charge. If your next purchase attempt puts you over the limit, that charge will be declined. At that point, you can simply call your resort and authorize another room charge to be started -- they are TOTALLY set up to do this by phone. They did this by design, to make it as easy as possible, especially nowadays when every self-respecting 4-year-old seems to have a cellphone.
When you reach your limit, THEN your credit card or debit card is automatically charged. And like I said, you can set up another charge and keep going. AGAIN, they only limit how much "charge" you have at any moment, but they have no want to limit how much you can room charge over time. SOmeone mentioned having $4200 spending money. You could spend 100% of this on room charges, even if you're at a Value with a $500 limit. Let's say in the first hour of your first day you charge $500 on Mickey bars for 250 of your closest friends. Just get on the horn to your resort and set up another room charge. It's really that simple.
As often as you like, you can go to your resort's front desk and get an up-to-the-minute printout of your account. If you see you are near your limit, you can decide to have them charge that amount to your credit/debit card at that time, and then set up a brand spankin' new room charge account so you won't be sidelined for a few minutes on the phone later that day or the next day to set up another room charge account.
Any other questions? Bring 'em on!