April 2011: Post updated to remove links to older Google Reader Notifier and link to official Google Notifier tool.
I'll admit it, I goofed. The screenshot for my post on using MisFox to set your Internet helper applications shows "Google Reader.app" in the pop-up of choices for the default RSS feed reader; this got several people quite excited. "Ooo! Can we set Google Reader as default? Can we can we huh?" they eagerly asked (I'm paraphrasing). Two teensy problems with my screenshot, though: first, the app listed as Google Reader is now known as Google Notifier; second, MisFox won't actually allow you to set Reader Notifier as your RSS helper app. Argh!
Fortunately, the answer is simpler than I thought. Go ahead and download the excellent and free GRN; launch it and put in your Google credentials, and you'll immediately get the Growl notifications and pulldown menu goodness -- however, if you don't want them, go to the Reader Notifier preferences and check "minimal function mode."
Now, in your browser of choice (works for me in Safari 2, Safari 3 beta, and Firefox 2) go to Preferences, go to the RSS/Feeds section, pick the Reader Notifier application (in FF, you'll have to click "Choose Application" and select it)... and there's no step 4. From that point on, clicking the feed link in the browser's URL field will automatically open the feed in Google Reader rather than in your local feedreader.
For the record, of the helper-app preference panes I tested, MisFox and MoreInternet could not set Reader Notifier as the RSS default, but RCDefaultApp did it with no problem at all (not to mention it has plenty of other functionality). Enjoy, and my apologies to everyone who got hot under the collar from the original screenshot.