While many organizations have replaced the dial-in connections with dedicated Internet connections, making war dialing not as relevant today as it once was, there are still devices out there that can be identified by using war dialing, so it cannot be underestimated as an attack method.

Non-writer edit: I see a lot of negative feedback on this question but, as a 12 year practitioner, I have to agree that there are still more remote-access modems out there than people would like to admit. Just last month I had a client show me that they had a modem and said it was for remote access to their router when there were clearly better remote access methods already configured on the network and not a single person in the business could tell me the phone number for dial-in. So... war dialing is still relevant, although it would be a pretty frustrating endeavor if doing it blindly nowadays.

EDIT: To editor, thanks for adding your perspective because the CEH study guide says it's relevant but us noobs don't have practical experience proving to us that this is true.