I was using svn polling once every minute (* * * * *) and the clean build option. My
checkout time exceeds one minute.

So what happens is...
1) A build is triggered, this deletes the workspace (clean).
2) The svn checkout begins (more than 60s elapse).
3) Hudson svn polling tries to query the workspace to see if there are any updates, but the
workspace is not done checking out from step 2. This causes the following message:

Started on Aug 19, 2009 12:00:54 PM
Workspace doesn't contain http://xxxxxxxxxxxxxxxxxxxxxxxxxx. Need a new build
Done. Took 0.2 sec
Changes found

4) A new build is scheduled even though there are no new changes. Goto to step 1).

This creates an endlessly looping build.

A quick fix would be to suspend polling a project if a checkout/update is in progress.

Alternately, you could query the repository directly (i.e. svn log --limit 1
http://svnrepo/code/some/project) and compare the result to the previous change which
triggered a build.

You can workaround this issue by changing your poll schedule to something less frequent
which will allow your build time to finish the checkout.