That's no small feat to pull off. Here's why.
Of the millions of feeds Google indexes an ever increasing number is full feed: the feed contains not a snippet but the complete item. Feeds are often (read: usually) limited to around 10 entries. After that they "roll off" the feed, being replaced with newly posted articles.
So... for Google to not only search but also show us the full article (or snippet) of a feed from October 7, 2005, Google has to have that content saved on their own servers.
Here's how you can leverage that enormous amount of data and incredible retention.
Better Site Search with Google Reader
A site I frequently search using using the [site:] operator is Lifehacker. And just as frequently I end up with the items I'm not looking for ranked first, followed by a bunch of comments.
Now the comment threads on Lifehacker can often be jewels of additional information and tips but it's hardly ever what I'm looking for. I'm looking for the content of the original posts.
Introduce the search in Google Reader. Where the [site:] search gives me well over 50 results, most of which I'm not interested in, Google Reader is able to bring it back to a spot on 4 results:
Now that is site search.
Better "Bookmarking" with Google Reader
The problem with bookmarking pages is two-fold. One, you never bookmarked that resource you now need so desperately. Two, if you bookmarked it, the page happens to be gone while you need it now.
Michael Gray writes:
"While I love delicious it lacks the ability to permanently archive a copy of the page like furl.net does, so if the page �disappears� or falls victim to linkrot, I don�t lose the data. None of the bookmarking services have good search functionality. Delicious lets you search across your notes, but you really need the ability to search the actual page/archived page contents to take a giant leap forward in use fullness."
The solution: the search in Google Reader.
Every post in your reader is permanently cached. You can find any resource back. Remember a part of that solid quote? Feed it into your Google Reader search and you have it in seconds. Need to read up on that press release that has disappeared for some reason? No worries: Google Reader has its own cached copy.
Bonus: Force your own permanent cache
Once a feed has really gone there's no way to add it to Google Reader. Be sure to add as many remotely useful feeds as you can: better be safe than sorry.
But better yet: protect "your" data and force your own Google Reader cache of a site.
Sign up at FeedBurner, login and create your own FeedBurner feed for the site you're interested in. As long as you don't delete that entry in your FeedBurner account the feed, and its data, will remain cached by Google Reader.