Dana triggered a thought...how can you be sure you are subscribing to the RSS feed you think you are subscribing to?
I'm not so worried about the scenario he provides where you go to Technorati and generate an RSS feed based on keywords (Watchlist)...'noise' is the worst that can happen.
I'm thinking about the scenario where bandits try to distribute phishing urls or other crap via an RSS feed that pupports to be from someone else (a bank for example)...So - Bandit sets up RSS directory with 99 bona-fide, useful RSS feeds, plus one dodgy feed. Unsuspecting user subscribes to dodgy feed. Feed gives initally good, bona-fide content for a month, then inserts article asking to you login and check details: 'Click here'. Viola! - RSS spoofing.
I realise this is a bit alarmist, and I haven't seen or heard of any examples of this. Just a thought.