Whilst using filetype:rss in Google will almost definately return valid RSS files it may not.

The reason for this is that Google's filetype search modifier (for non supported files, such as PDF Word Docs, Excel spreadsheets etc.) works on the file extension. Although it is unlikely that anyone would use the rss extension for anything other than an RSS file it is possible.

More importantly though is that most RSS feeds are auto generated by server side code and don't use an RSS extension.

.php .cgi .asp .pl etc are all more likely so that's why I prefer Yahoo's search over Google's for finding feeds as their advanced search works on the file content rather than file extension

Jason D