This is a great question because it's something that every publisher views differently. Speaking from my own personal experience, I've had publishers that insisted on every single image having written permission before we could publish it, and other publishers that have said we should just go for it, as long as there aren't any explicit terms of service or copyright notices that prohibit reproduction.

As a result, many of the screen shots in my book Creating Cool Web Sites are of my own properties (far too much hassle to get screen permissions for dozens and dozens of sites), while my book Growing Your Business with Google has lots of images, including some that are used as the basis of criticizing the company in question: that publisher agreed with me that material in the public eye is fair game.

There are also nuances too. Once you decide you want to get permission, what permission are you really getting? And what do they need to know before they'll approve your use?

In one book I wanted to use a customer service page from the Toyota Motor Corporation Web site, and when we communicated with their PR team, they told us that they wanted to first review the entire manuscript to ensure that the use was favorable. Obviously, I didn't agree to that and I axed their placement in my book, substituting a different, more friendly firm.

Remember also that it's legal and defensible for you to have an opinion, informed or otherwise, about another site, property or company. Saying "I think Frito-Lay sells garbage to kids" isn't legally actionable, nor is "I think that a senile half-blind monkey could produce a more attractive Web site than the firm that built MTV.com".

Where things become more risky, where you move into the world of libel law, is when you state facts that are incorrect. If you say "After having hired a white supremacist organization to develop their site, Company X also made sizable donations to Abu Nidal before issuing a press release about its "green" and "socially responsible" policies" then you darn well better have irrefutable facts to back that up.

I think this would be true also with Web pages that you publish. If you're using them to illustrate a point, you're totally in the clear. If you're using them to illustrate your point or opinion, again, it's hard for the company to complain too much, but if you're directly criticizing the company itself, you might want to be a bit careful.

I hope this answer is helpful!