The relationship between Apple and Twitter seems very good to me, so I think the reason Apple doesn't use pull-to-refresh is that they want to be conservative in their UI rather than they are afraid of a patent lawsuit from Twitter.
I think that's a very optimistic take on the situation. Given the amount of VC money sunk into Twitter, and the amount of cash Apple has, I think it's a prudent move on Apple's part to stay well clear of infringement.
I feel that replacing the word Twitter with the word Google and moving the clock back to 2007 would make that statement just as valid... and just as dangerous.
