With any of these big companies like FB, I think you're good to go right up till you're not. For me, I'm not really interested in pushing the boundaries with the cover image because 1) most people probably visit a page once, to click like, and then never return, and 2) I don't see my FB page as a viable lead capture tool. For the most part, the people who have liked my page are clients, prospects, people I've met at trade shows or networking events, and so on. They've already gotten a peek at what I have to offer, so my job is to use the ability to put my message in (16% of) their newsfeeds to stay top of mind. That allows FB to be a tool for generating referrals from fans, but cold leads? Doubt it.
Oh, and since we're sharing: www.facebook.com/RevolutionaryGardens