Twenty years ago intellectual property was a different beast. Creators would put a big chain link fence around their work and say bluntly "back off!" to anyone who tried to spread their ideas. Today, the landscape has changed. I remember sitting down with one of my clients to discuss her blogging strategy. I mentioned a quote I heard from I believe Seth Godin, but please correct me if I'm wrong - "Blogs are a way to create conversation, not control it." With that in mind, the strategy becomes referencing other blogs, commenting often, linking strategically and sprinkling in your own opinion. This new philosophy was one my client didn't really get. She became concerned with bloggers stealing her ideas or being hunted down by the copyright police if she linked to another blog without their permission.
This is a fine line we walk. Andy Sernovitz posted some excellent insights into this topic.
"'What if someone steals our stuff?' is the wrong question. Ask 'How can we get people to steal our stuff?'...When we advertise, we pay to spread our content. Don't stop customers from doing it for free!"
I have to say I agree with Andy. Make your ideas useful so your clients will spread the love. Of course as Andy points out, "Do insist that your content is properly credited, with a link to your site, but beyond that ... encourage the sharing."
As for blogging, it's proper etiquette to link to the other blog when you are quoting (like I just did with Andy). You need not ask for permission ahead of time - that's a part of the culture we've created. Linking helps them grow their blog. They will be glad you're sharing their ideas with your readers. You would be hard pressed to find a blogger that says "No - I must protect my intellectual property. I don't want your link."