<p>I'm trying to get some stats on how many of the visitors to our website have Silverlight enabled browsers.  </p>

<p>We currently use Google Analytics for the rest of our stats so ideally we'd like to just add 'Silverlight enabled' tracking in with the rest of our Google Analytics stats.  But if it has to get written out to a DB etc then so be it.  </p>

<p>Nikhil has <a href="http://www.nikhilk.net/Silverlight-Analytics.aspx" rel="nofollow noreferrer">some javascript</a> to Silverlight tracking to Google Analytics.  I have tried this code but Google Analytics doesn't pick it up.</p>

<p>Does anyone have any other ideas/techniques?</p>

I think you answered it yourself. The page you are linking to does just that: detect which version of Silverlight the user has (not if s/he installs it). From the page:
After a little poking around, I found that Google Analytics has support for reporting a user-defined field.
...
Basically this detects the presence of Silverlight, and if its available, it records the version as the value of the user-defined field. Now your analytics reports will have one of three values: "(not set)", "Silverlight/1.0" or "Silverlight/2.0".