There's a lot of speculation, opinion, and incomplete information floating around this thread. Please allow me to present the FACTS about room charges ... not my opinion, not based only on my experiences, not what I hope is true, not what I heard from a bus driver, but THE FACTS:
First of all , you don't need a credit card ... or even a debit card. At WDW resorts, a room charge can be set up with a credit card, debit card (with VISA/MC logo), travelers checks, cash, or even Disney Dollars.
If you use a credit card or debit card, they place NO HOLD on that card. None whatsoever.
I understand that many other places might do this, but it's 100% fact that WDW does NOT do this. When you present your plastic (debit or credit) for the purpose of setting up a room charge, they swipe the card and run through a charge in the amount of zero dollars and zero cents ($0.00). They do this in order to make sure the card is linked to an active account (it will not be approved if the card isn't "real" or is based on a closed account). Also, they don't check your available credit or bank balance, and once again, they place NO HOLD whatsoever.
So once again ... NO HOLD on a credit or debit account. NO CHECK OF YOUR CREDIT LIMIT OR AVAILABLE BALANCE. Period. Fact.
There ARE upper limits to room charge by level of resort, as reported earlier by CR Resort Fan. He is correct -- $500 for Values, $1K for Mods, $1.5K for Deluxes. Of course, if you meet that limit, you can set up another one! Disney protects themselves by setting these limits, because they never checked your card's available credit/available balance when you checked in. If you only have $20 in your checking account and you plunked down your debit card to set up the room charge, when you reach the $500 limit at a Value they will then attempt to charge the card, only to learn that you don't have the money to pay for it. Disney would rather chase after you for $500 than for $2000 or something like that.
So there are upper limits. You CAN set a lower room charge limit for yourself if you desire. Some front desk CMs don't know this, but that's just inexperience on their part. They might ask a lead or manager if they don't already know this, but you can be sure of yourself that it can be done. It does not require a manager's OK to set up a $852 room charge at a Moderate resort where the upper limit is $1000. A lot of times, this is a sort-of "don't ask don't tell" situation. Guests don't ask about the amount of the room charge, CMs don't mention it, and thus guests who've experienced this several times find it to be a sort-of random experience and don't recognize that there is, in fact, a pattern (WDW policy) to it.
Your card will NOT be charged until one of the following events occurs: you meet/exceed your room charge limit; or it gets to be about 3:00 a.m. on your check-out day. At any time before your account gets charged to your card, you are free to go to the front desk to pay off some or all of your account by other means. You could do this every day if you wanted to. The only important part to know here is that if you want to do it, you MUST do it before your room charge account meets/exceeds the limit you set at check-in.
Once you meet/exceed the limit, you can simply set up another room charge. In other words, when they set a $500 limit at Value resorts, that's not a $500 limit for the entire vacation, it's just a $500 limit until a new room charge account is set up. You can set up a new room charge by phone if you want -- you don't even need to go to the front desk to do so. Some resorts, or rather some CMs, will automatically set up a new room charge for you once you meet/exceed the limit. This seems to vary on a case-by-case basis.
If you put down cash, travelers checks or Disney Dollars to back up your room charge, and you did not meet/exceed your limit, you will receive an immediate CASH refund when you check out.
There can be only one room charge account per room. In other words, two independent adults sharing a room can't EACH start a separate room charge. There can only be the one room charge for that room. However, Disney keeps track of which exact Key To The World card charged which purchases, making it possible for roomies to easily divvy up who owes what on the room charge. You can go to the front desk at any time and get an up-to-the-minute printout of your room charge activities. It will be specific by date/time/location of purchase, as well as which KTTW card number was used to make the purchase; the only thing it doesn't do is itemize items within each purchase -- in other words, if you spent $100 at 1:00 p.m. today at the Emporium buying 5 $20 shirts, the printout will show a $100 purchase at that store at that time, but it will NOT list each individual item within the overall purchase.
As an unrelated FYI, it's important to note that, room charge or not, WDW resorts do not require you to present a debit or credit card at time of check-in to cover "incidentals" or anything like that. If you wanted to, you could do WDW totally plastic-free.
) and every time we check in we have a $1500 credit line no matter where we stay. Back in the 90's my DH had an AmEx card and the limit when we stayed at the Contemporary was $3000! I nearly fell over and said don't even think about it! 
Now we use our debit card and we put charging priviledges on one card. They've never put a hold on our money, because we would have tons of things bouncing while we are gone since our bills are set up to be paid automatically. This time we were able to upgrade to an annual pass and use his room card to do this before we left. We plan to return in December for our 30th Anniversary! 
