Auto companies tear down each others cars. Tech companies tear down each others products. Grocery stores check out each others prices.
and so and so on.
None of them steal. Just poke, prod, look, and learn. Then apply to their own if it's suitable.
I have never seen a Non-compete clause in any TOS.
http://techcrunch.com/2010/10/27/myspace-pinterest/
This goes a bit beyond what you're talking about as it's not just checking it out but blatantly imitating. That said, the fallout is likely to come down on My_____, at least amongst the technorati.
Just keep in mind that you don't want to be too focused on what your competition is doing. Its ok to check them out, but focus on your customers. They are the ones that really matter.