Stephane Rodriguez posted an interesting comment that I hadn't thought of - it isn't the easiest thing in the world to download Orca. You have to go through the Platform SDK web-based installer tool and also install a bunch of pieces of the core SDK to get orca.msi which lets you install Orca. I have a copy of it that I downloaded and posted at http://cid-27e6a35d1a492af7.skydrive.live.com/self.aspx/Blog%5E_Tools/orca.zip for easier access. I hope this makes things easier for you as you debug your setup packages.
<update date="4/9/2010"> Fixed broken link to orca.zip. </update>