There seems to be a scarcity of good quality weblogs about racing, but I just found Full Throttle which looks good. (Their RSS feed is not linked from the main page, but with a little digging I found it: Full Throttle RSS feed.)
I also found Sports Blog which is kind of a meta-weblog, bringing in reports from a bunch of authors on different sports, including racing.
I’ve made my own feed (scraped) for Jayski’s Silly Season which is a really good NASCAR source. I need to propose that they put out an RSS feed as well.