If you are experimenting with ClickOnce in Longhorn, here are couple tips.  First, the following article is essential:
http://longhorn.msdn.microsoft.com/lhsdk/appcore/overviews/appmodel_life_cycle.aspx
This page has a script you need to run on IIS to enable it
for ClickOnce.  Second, if you are looking for the ClickOnce log file, it is located at:

%USERPROFILE%\local settings\my
applications\system.deploymentframework.log

This can be helpful for debugging.