This is a grey legal area, but the legal precedent is that if you (intentionally or otherwise) do harm to a third party's business by linking, framing, inlining or otherwise misrepresenting their content they have a case for defamation.
Danny Sullivan has a great page up on META tag lawsuits:
The obvious cases like the Washington Post vs. Total News seem clear-cut; Total News framed up the Washington Post site and surrounded it with ads in their own frameset (deriving revenue from WP's content).
But others are less obvious - Ticketmaster sued Microsoft (Sidewalk) for deep linking into their site - bypassing the home page which contained their sponsor placements and main advertising. They sued under trademark dilution and unfair competition and settled confidentially. Ironically Microsoft later sold the entertainment section of Sidewalk to Ticketmaster...
Some choice quotes from "Legal Issues on the Net - Hyperlinking and Framing - [dlib.org...] " :
"Framing thus is less likely to survive legal challenge than generic hyperlinking. While it may or may not give rise to copyright liability, it is quite likely to run afoul of the trademark laws."
"Linking... in the absence of other wrongful conduct, is unlikely to constitute either copyright or trademark infringement. However, if a site were to imply an association with the linked site expressly or implicitly through the manner in which it uses the trademark, it may be liable under the Lanham Act."
Another from "Nolo's Legal Encyclopedia - [nolo.com...] "
"Copyright Infringement. Although it is not a violation of copyright law to create a hyperlink, it is a violation of the law to create a link that contributes to unauthorized copying of a copyrighted work if the linking party knew or had reason to know of the unauthorized copying and encouraged it."
This is something I've researched before as occasionally you get requests to remove pages that infringe on trademarks (often accidentally) and also I have had clients that "misappropriated" copyrighted content and gave it to us as their own copy for inclusion on a site;) Always a joy.
Links for further reading:
META tag lawsuits
Open Directory Category on Linking Law
Hyperlinking and Framing
Unlawful Linking and Framing
Linking, Framing, and Inlining
Link Law: the emerging Law of internet hyperlinks