Subatomic are a sneaky bunch of folk who, somehow, found the time to slip in a few hidden little artistic extras.
If you take a look at the rock face leading in to Lavaflow area, there's the face of a couple of well-known Fieldrunners carved into it.
On the Lavaflow map 'The Frying Pan', if you look at the top right-hand corner of the screen, you'll see these face carvings a lot more clearly...
A few of the designers have had shops opened up in their name in-game. Jim's cafe can be found in the 'Last Limbo' and 'Be Mine' maps, Serafino's shoe store is located on the 'Bizarre Bazaar' map and 'Broken Branch' has Canfield's farm.
Once you finish a level on any difficulty, you'll unlock a director's commentary for it. Simply choose the clipboard when you replay that level to hear the developers talk about how they made it!