A while back, we added identi.ca messages or "dents" to our search index. It was and still is accessible at a second search page(tweetscan.com/identica). Since then, we've received requests that it be integrated to search both Twitter and identi.ca in one search result.
We've taken this idea a step further by integrating not just these two major Microblogging sites but also eight other Laconica based sites. These sites are in no particular order: bleeper.de, army.twit.tv, lareta.net, linuxoutlaws.net, sufficetosay.frontrowcrew.com, micro.ciarang.com, fun.ohhh.no, and pawwer.com. None of these gets the volume approaching Twitter at the moment but we think it's a great thing for the microblogging space to have them somehow unified in search.
For the moment, RSS doesn't do all these sites but you can do Twitter and identi.ca by visiting the API page to read about that. Also, if you know of another major site we should be indexing, feel free to comment or message @tweetscan or @weex.