Note that one downside to the Signature card is that since it's a "no preset spending limit card", many banks that issue it do not report the credit limit to the CRAs, which can negatively impact your credit utilization, which in turn can affect your credit score. Much discussion on this topic at cardratings.com, fatwallet.com and creditboards.com

http://www.bankrate.com/brm/news/cc/20060331a1.asp