I hadn't heard of the second possible attack that you mention until now. Could you please point me to an article or somewhere that develops the idea. I'm intrigued about what people predict that the miners of such pool would do in that scenario.
The interesting point about the second possible attack is that a weaker form is possible even with less than 50% of the hashing power. The term to search for in a search engine is "selfish mining", e.g. here: http://bitcoinmagazine.com/7953/selfish-mining-a-25-attack-a...
