Wikipedia is important for SEO and perhaps more importantly, for online PR (E-PR) since:

• Dominant within Google SERPs (Google's biggest downstream site according and Wikipedias biggest traffic source according to Hitwise)
• Relevant links within introductory Wikipedia articles can generate significant visitors
• Most large brands have pages created for them which you need to monitor and respond to what others are saying about you.
  

It is less important than it was for inbound links since it implemented the nofollow hyperlink attribute in 2006 which means that PageRank / link juice is not passed onto the destination site for the link.

Here is an excellent post on SEO for Wikipedia. It reviews the main tactics you may want to try including:

• Building a reputation based on adding notable content
• Partipating in the discussion for the page
• Considering developing a separate persona from your official role to post
  

E-consultancy has a more recent discussion on learning best practice in SEO from Wikipedia's own best practice

Finding out the number of visitors to Wikipedia pages

You can see page popularity in Wikipedia with a newish tool which threatens to "disappear at any time". For example, you can see that the page on "SEO" gets around a maxium of 500 views per day.

• See Number of visitors to Wikipedia pages
• See top pages in Wikipedia

Since Wikipedia is in a top 3 position for many searches, it can be used to gain an idea of how important Wikipedia is, although different search terms will refer visits to it of course and then there are browsers.