With Mac OS X 10.4 Tiger, iCal events that you download using .Mac Webmail will have an HTML (.html) file extension instead of an iCal (.ics) one. Because of this, when you double-click the file, iCal won't open and instead, your default web browser will try to open the file (or whatever application you've assigned to open .html files).
If this happens, simply change the ".html" file extension on the affected file to ".ics." For example, if you download an iCal 1.5-sent invitation event named "mydates" using .Mac Webmail, the file will be saved as "mydates.html." To make this file open in iCal when you double-click it, change its name to "mydates.ics."
Note: Mac OS X 10.4 includes iCal version 2.0, which is the version that will open when you double-click an iCal file.