Having done almost this exact thing, the issue is that 4 out of 5 large websites will have blocked you buy your fifth poll for abusing their service.

I always thought that it was somewhat ironic that nobody had a problem with me refreshing slashdot.org every 10 seconds trying to get a first post, but that they had a very large problem when my rss poll hit them more than once an hour (is at least what their recommended interval used to be.)