Today I IP-banned my first RSS-abuser. He was polling my Something Awful feed about every few seconds, for long stretches of the day.
Folks, there's data in RSS about how often you should poll, and don't poll more often than that. If you're in a hurry to implement your reader, it's a snap to implement just the "ttl" field until you get around to the whole thing. This is on top of making your reader use If-Modified-Since headers.