Obfuscation via a suitably-long unpredictable URL can work, but as others have noted, there are a number of ways such an URL can leak -- toolbars/browser-plugins being one of them. Also, that a user can easily forward the URL to others to grant access may be a bug or a feature, depending on your preferences.

One often overlooked leak: the 'Referer' [sic] header. If your document is hypertext, and includes outlinks to elsewhere, and the authorized user(s) click those links, those outlink-target sites may receive your confidential URL as a 'Referer' header. As some sites then publish their referrers, in one way or another, the 'secret' URL could wind up in public.

Remember this before creating a 'Competitors' page with outlinks on a 'login-required' but plain-HTTP wiki!