Perhaps you haven't heard the news.... the economy isn't doing so well, and there's a credit crunch. A lot of credit card issuers are cutting limits or outright canceling cards, even on people who really seem to be low-risk.
The Chase World MasterCard has a published benefit of a "no pre-set spending limit." You can exceed your credit limit with no penalty, as long as you pay the full amount over your credit limit (plus your standard revolving charges' minimum payment) when your bill comes due.
In this way, MasterCard (as well as Visa's Signature product) competes with American Express's charge cards.
The amount which the issuing banks will allow you to exceed your limit is based on a number of factors, one of which is your previous spending and payment patterns. If you have a $10,000 limit but usually only charge $1,000 per month and pay it off in full but then suddenly charge $10,200 in another month, the issuing bank may decline charges after a moderate amount over your limit--maybe $1,000 or $2,000. However, if you have a $25,000 limit and regularly charge $20,000 and pay it off in full every month, the bank may feel comfortable allowing you to exceed your limit by $10,000 or more.
The worst case: if you have a limit of, say, $5,000 but have a habit of charging $1,000 but only paying off $200 per month and reach the $5,000 limit after just a few months, the bank is not going to let you go much more than a few dollars over your limit before declining charges. And if you've missed payments, you can probably forget about going over the limit at all (and the bank would likely drop you down to a more basic card which does not include the "no pre-set spending limit" feature).
The economic slump is definitely causing the card issuers to cut the amounts they'll allow cardholders to go over their limits, so whatever they used to allow is now cut down to a fraction of that.