I added support for static calendar urls. For right now you have to edit the script manually (see the notes section). I will work on updating/expanding the settings to make that part easier, but I wanted to get this out sooner.

Keep in mind, if you use this method, you will have to edit the script whenever it is updated here. And while using bookmarks means that your stored calendars are accessible across browsers/profiles, local storage is not.

More on the subject of google bookmarks:
I could not find any references confirming that google bookmarks are public. In fact, I could not even find a way to share them. From everything I read and in my testing, you have to be logged in to view your bookmarks.

@tonyaross
I guess my response was a little vague. I knew what you meant, I am just referring to it as the rtm feature for convenience.
I did some testing, and it is both easier than I thought (though still more complicated than the entirety of the script as-is, heh) and more useful. So I will get to it, I just can't give you an estimate of when.

I don't use rtm at all, so any sort of real integration with that is not anticipated.

@Forest Gregg
The 'except on' events are now handled like 'canceled' events - they are still displayed, but with a strikethrough font. If you would rather they not show up at all, let me know.

re:Private Links (or bookmark-less calendars)
This idea is growing on me. It will be in the next 'major' release.

Re: Remember the Milk
This would be a large undertaking and would have to go back to being a hack rather than using the gmail api. While I like the idea, it probably won't happen anytime soon.

Re: Private links
It only uses google bookmarks as an easy place to store the list of calendar urls. You could fairly easily modify the script and hardcode your list of calendar urls. You would want to change the 'refresh' function to call 'load_calendar' for each calendar you want to load rather than 'load_bookmarks'.
I may consider adding direct support for this at some point in the future, but it is a low priority.

@tonyaross - I like that idea. I will look into it. But I make no promises :-)

@ronr - Nothing strikes me as obvious. If it makes it past 'retrieving calendars' then it is connecting and downloading, it just doesn't find anything to display. If you have firebug installed you can see a bunch of status messages that might reveal something.

This script has been deprecated. I have rewritten it, making use of the gmail api and (hopefully) making it more maintainable. The new script is called Gmail Calendar Nav Module and can be found here. Enjoy!

I noticed your comment on the 'sidebar' script regarding timezones. I am not sure what changed to make it stop working, but I am looking into it. I should have something soon - probably by the end of this weekend.

Hey Brad, I've made some enhancements to this scripts. I cleaned up the caching so it doesn't have to re-render every time gmail is refreshed - my computer is slow :( ; I moved the color checking to the name of the bookmark rather than the name of the calendar ; and I started pulling the bookmark data from the bookmark rss feed ; and a couple of other minor fixes to prevent firebug from spamming me with warnings :-)
I'm wasn't sure of the best way to get it to you (since I couldn't find an email, or private message thing with attachments) so I just posted it as a new script. Review it at your leisure, incorporate the changes - or not :-)
Anyway, here it is: http://userscripts.org/scripts/show/12396