Hi Berner,
Good question and some thoughtful answers here. I'll chime in with my 2 pesos.
A citation is defined as a web-based reference to a local business' complete or partial NAP (name, address, phone). A citation does not have to include a link the company website. For example, a blogger could mention your restaurant's name and address, leave out the phone number and not link to the restaurant's website and that would still count as a citation, meaning that the business owner must be certain that the blogger has accurately published the restaurant's name and address.
On more standardized platforms, like local business directories, nearly all allow you to include a link to your website. This does count as a link, but whether it is nofollowed or not is up to the individual directory. So, this does bear on how much 'juice' a given link is passing your way.
Regardless, if your business is a local one, citations are a core part of the work you will be doing to promote your business on the Internet. If your business model isn't truly local, then citations aren't really meant for you.
I think you'll really enjoy reading the great section in the new Moz Local Learning Center that covers the ins and outs of citations. Tons of great resources here for you: http://moz.com/learn/local/listings
Hope this will help, and kudos to all on the good responses on this thread!