Tom Lane committed a patch for index-only scans by myself and Ibrar Ahmed, which also incorporated some previous work by Heikki Linnakangas, after hacking on it some more himself. Woohoo!
There is, of course, more work to be done here - performance fine-tuning, cost estimation, extensions to the core functionality - but the core of the feature is now in. If you get a chance, please test it out and let us know how it works for you.
For those that may not have been following along at home, what we're essentially doing here is allowing any index to act as a "covering index". If all of the columns the query needs are available from the index tuple, we'll skip fetching the corresponding heap (table) page if every tuple on that page is visible to all running transactions.
Although I know we're not even really done with this feature yet, I can't help wondering what's next. Index-only scans have so often be cited as "the big performance feature that PostgreSQL is missing" that it's become something of a cliché. Now that we have them, what will take their place as the next big thing?