The main build status page shows a 48x48 icon representing the build status. If you visit the page during the build this shows a flashing "build in progress" icon. However when the build stops the "build in progress" icon continues to be displayed.

This is due to browser caching behaviour. For example the recent build at the following URL

http://ci.jenkins-ci.org/job/tools_maven-hpi-plugin-maven-2.x/40/

During the build the relevant markup was

<img height="48" alt="In progress" width="48" src="buildStatus" tooltip="In progress" />
        Build #40
        (23-Mar-2012 01:18:30)

The request for buildStatus returned a HTTP 302 redirect to

Location:http://ci.jenkins-ci.org/images/48x48/blue_anime.gif

The combination of a 302 redirect and caching headers in the gif cause the browser to not re-request buildStatus within the same browser session.

http://ci.jenkins-ci.org/job/tools_maven-hpi-plugin-maven-2.x/40/buildStatus

Note this is unrelated to the recent changes for caching of images etc. I have noticed this in the past but have not had time to investigate before now.