With cash sitting in my account, IB sent me a margin warning - and I have no margin. Apparently I'm within 5% of exceeding my margin, which means that if I take a dollar of margin, I'll be margined out. Admittedly, I have a very concentrated account (in two stocks), but I think IB might want to wait until I have a dollar in margin before they send me a margin warning.
The link below may assist in better understanding why you received this message. If not, please feel free to send a private message and we can look into the specific details of your account. http://ibkb.interactivebrokers.com/node/226