The Eighth Circuit has decided that cow spots are, well, cow spots—at least in the computer industry. In Gateway , Inc. v. Companion Products, Inc., No. 03-3410 (13 Sep 04) (slip op.), the Court determined that Gateway's black-and-white-cow-spotted boxes had acquired secondary meaning in the computer industry, or at least among end-users, and therefore could block Companion Products from distributing a black-and-white-spotted-cow "stretch pet" toy intended to wrap around a monitor.
The problem with this decision is that it contains a lot of broad language that is easy to quote out of context. There is one critical fact that seriously limits the opinion's proper scope:
Interestingly, Cody Cow's creation is related to Gateway's use of black-and-white cow spots in its advertising. Because Gateway uses black-and-white cow spots in its advertising, CPI's president, Dennis Byer, proposed creating a product for Gateway. Byer considered Gateway an attractive potential customer. Byer produced a document featuring Cody Cow, with a black-and-white cow-spotted design, wrapped around a computer monitor that displayed the wording "an idea for Gateway." On June 1, 1999, CPI sent a sample of the Cody Cow character and a letter to Gateway stating that "our initial sample was a black and white cow designed with Gateway in mind." For its second sample, CPI used a stuffed, black-and-white spotted cow that Byer had bought from a nearby Gateway Country store. Gateway rejected CPI's offer and informed Byer that Gateway had a registered trademark for its black-and-white cow spot design as it related to computers.
Slip op. at 3 (footnote omitted). In other words, the defendant not only knew about the mark, but had previously attempted to form a business partnership for exploiting the mark and been rejected. It is unclear from the opinion whether Gateway simply had a different exploitation of this nature in mind or rejected the entire category, although there is a hint that the "stretch pet" was just an extension of an existing Gateway-provided plush toy.
This presents some interesting questions on the difference between trademark and copyright law. In copyright law, a parody can still be fair use, even if permission for the parody had been requested and refused. Of course, copyright enforcement is selective—that is, one need not attack every infringer—while trademark enforcement is "protect or lose." The question then becomes more one of similarity than of legal duty, and this is inconsistent with the literary principles behind parody. In literary terms, the looser (less connected) a parody, the less effective that parody generally is—and that is without considering the issues of satire and the Ninth Circuit's ill-considered "target" test. 
Consider, too, what might have happened to CPI if its cow had been more "naturalistic" instead of cartoon-like. Assume for the moment that its stretch toy was made from real cowhide in a black-and-white-spotted pattern and modelled to realistic proportions (at least out of the box, before stretching it around the monitor). One can make a much better argument that this is a "parody," and therefore should be protected, than on the actual facts; it implicitly points out that Gateway's mark is wholly artificial and cartoonlike (which, given the quality of Gateway's documentation, would be a substantial improvement). Absent the permissions issue, what should a court do if Gateway filed an otherwise-identical lawsuit? Hint: It's not even as clear as this description makes it seem.
What this really does, more than anything else, is point out that trademark and copyright treat parody inconsistently. One might well wonder whether the Mad Hatter ("In This Style") had anything to do with writing either, or both, statutes. Referring to Alice is probably more appropriate in this context than it appears at first blush: Recall that the Alice books are themselves on the border between satire and parody, and further Humpty-Dumpty's redefinitions to meet his own purposes… but that's far enough into the murky and dangerous depths of literary theory for one day. Perhaps even for one lifetime.