This type of site benefits from users actually filling information in. The more drinks entered by users the more the site is of interest, and the more it grows.
Therefore, even for returning users, isn't the primary aim to get users to enter what they are drinking?
Just because a returning user knows what to do does not make your call to action any less useful. In fact, in a sense it's even more necessary. Returning users might be coming back with the intention to just see 'what's going on', therefore, a call to action like the second one says to the user: 'Hey - before you look at the rest of the page, you might want to do this.' It gives you that little extra kick towards fulfilling your primary aim of getting users actually putting information in rather than just browsing.
Additionally, the position of the entry fields on the page is where users are very used to finding a search box. The large call to action that you have in option 2 clarifies what this area is about and makes it clear that it is not a search area, but the primary interaction area - in a similar vein to Twitter's "What's happening?".
Of course, as Rahul says, you might want to try both and see just what effect the second option has to your users on your site (and let us know!?)
Finally it's good to make the call to action clear for mobile users which might well comprise a good section of your target users? - For example here's a good approximation of how it looks in my blackberry with a 320px wide screen. (Using Opera Mini)
For mobile, it is necessary to be very obvious because screen real estate is a premium and the message needs to be clear, obvious and readable. The less obvious option using the call to action via light grey text inside the text field is not sufficient for mobile.
On mobile, I can at least clearly see which area of the screen to zoom in on, because 'What are you drinking' is the most easily legible part of the screen.