I can confirm this to be true because they also cut down my credit line.  Here is a post from Yahoo that pretty much sums up what they did to me (except I didn't even get a phone call - was a letter):

I just received a shocking call from American Express.

My small business has had an American Express Business Account for the past two years. Our credit limit was around $25,000 and our average bill was approximately $12,000/month.

We have NEVER had a single late payment and, according to Amex's customer service reps, our spotless payment history is considered "perfect." In fact, most of the time, we pay our full bill prior to the date it is due.

So imagine our surprise when Amex called us today to inform us that our new credit limit on the account was $1,800. When pressed for details, the Amex rep made some vague references to a credit report.

However, our credit report is spotless. The only possible factor could be the fact that, since we are a private company, we do not share our financial information with Dun & Bradstreet.

Our company has bank lines worth several millions of dollars. We have been a loyal Amex customer. In an era where defaults are soaring through the roof, we have consistently paid our bill in full and prior to its due date. Doesn't that count for anything anymore?

Apparently, not with American Express. Considering that Amex spends a huge fortune wooing small businesses and marketing their Open Services, it seems that this is just another case of hypocrisy on the part of American Express, a clearly beleaguered financial institution.

I just wanted to share this information so that other small-business older are not blindsided by Amex's shocking display of customer disloyalty.