Yes, even hardcore rivals can be social-media friends (or followers, at least). Earlier today Amnesia Razorfish launched an experiment to see if it could get the cola brands to follow each other on Twitter. It tweeted the original challenge and then documented the results and the full story of how it all went down on its blog.
Coke was first to say a "gracious (but competitive) hello" to Pepsi and follow its rival. Later Pepsi responded with its own greeting, tweeting "Can rivals and tweeps coexist? We're willing to find out. :)" Both are now following each other. You know what they say about keeping your friends close and enemies closer.