Let's start really tearing the details of this thing apart and really deducing what each and every tiny thing might MEAN
Remember, though we all love Rian dearly, authors intent doesn't really matter in a discussion on symbolism. The story is out there, and it's our job to interpret, draw conclusions from and find meaning in it. Some interpretations may make more sense then others, but simply hypothosizing on what the author meant would be rather boring, and a clear "right or wrong" argument. I'm more interested in what everyone is reading into it.
For instance, there's been some discussion on the multiple shots of swirling cream in coffee, including a ying/yang reading and a possible nod to technology from Douglas Adams universe. Personally, I saw it as a depiction of the way Old Joe is experiencing memory - once he's back in 2044, his memories become a cloud from two different sources - what he experienced, and what is actually happening. They're coming together, and hard to interpret, but eventually, they become one clear thing, that is, the present truth.
Other possible symbols: Is there a symbolic significance to the clocks, other than "it's time travel movie, time is clearly important." Everyone seems anxious, checking and watching the time, Joe ropped a watch shop, clocks are all unique and clearly placed.
Also, when Sara is is having her post-sex cigarette, there's a close up of a white rabbit ash tray. White rabbits are pretty iconic, due to Alice's Adventures in Wonderland, though their use may predate that. In Wonderland, the White Rabbit is running late, and in some versions, carries a broken pocket watch - again, significance on time and time pieces. But why associate Sara with the white rabbit? Theories?