Great work on the details!
Where's the light source in this one? Judging from their shadows I say it's from behind them, so you could've made the room a little darker and they're shadow longer and more detailed. The shadows are the only things that I immediately took notice of, nothing really much wrong with this one. I saw that the bars weren't casting shadows, too. (What is it with me and illumination physics?)
The idea is great, makes one think of how it's like to be behind bars.*ehem* I noticed that they're somewhat smiling, that really creeps the hell out of me because I don't like being in jail but I think they, being together is something to be happy about.
Once again, excellent job. You have a keen eye on detail. *look at it!*
UPDATE:
It looks more life-like now. You should really take into consideration some real-life aspects so there'd be more realism in your work.