Hello,

The bad news is you can't implement this. Google does not offer this as a service. It is another 'benefit' of having a highly optimised website that attracts quality traffic and gives relevant content to customers. 

If you look at the websites that have a search bar, they are generally large brands with high traffic volumes, highly optimised content and a large link base e.g. Argos.

We've been hunting for an article that gives us a convenient 1-2-3 of what a site has to achieve for Google to provide this level of functionality within natural search results but it's proving to be harder to track down than Lord Lucan.

Here is an online discussion around this subject:

http://www.google.com/support/forum/p/Webmasters/thread?tid=2503e0206087811d&hl=en

So all you can do is beaver away with your SEO and try to make your site as wonderful as possible in the far reaching and ever changing eye of Google.

thanks

james

PS If anyone can shed light on this I would also love to hear more information!