Is the ZenPage plug-in only available from SVN? I simply cannot find a download link for it.

Sorry, forgot to specify. I meant separately, as a plug-in. I see it comes included in the standard package, but what if I deleted it and wanted to re-install it, or if a new version of ZenPage was out but not for ZenPhoto.

Zenpage is an official plugin that is included in the release package and not available separatly. Zenpage is always tied to the current release of Zenphoto so there will be no update separatly as well. It is not recommended to install parts of the svn with another release. If you deleted it somehow just download the release package again.

I am usinf zenphoto 1.25 on my website http://www.pictagging.com do I require to just enable plugin to start zenpage? or I need to download latest plugin release to work?

You additionally need a theme with Zenpage support or have to add it to your theme.