This is a difficult
passage to interpret for numerous reasons...
- This particular story lacks any comparison in the rest of the gospels. Most biblical exegesis involves seeking out similar tales in the other texts we have in order to find the "truth" somewhere in between. This is a difficulty not only for this story, but for much of the Gospel of John and is one of the reasons the Gospel of John has been mostly discounted for the purposes of modern theological discussion on the life, historicity or meaning of Jesus.
- Compared to the other gospels, John tends to be very direct and straightforward in his telling of Jesus' tale. What few occlusions John leaves nearly always contain a Messianic subtext to them. The fact that John would take the trouble to mention Jesus' writing but refuse to state what exactly was written should be a red flag. Most likely this inclusion by John amounts to little more than a symbolic gesture. Since I see you've read your Old Testament - I'll point you to Exodus 31 where God inscribes the tablets with nothing save his finger... These two passages are frequently sited together to show John's interest in making Jesus' Messianic status overt.
- Most troublesome about this passage though is the very likely possiblity that it was not an original inclusion. All of the very earliest pericope exclude John 7:53-8:11 entirely.(1) This, coupled with the already problematic fact that John was probably not written until at least a generation (if not more) after the Gospel of Mark, makes the likelihood of any degree of "truth" to the story almost nil.
...When all that's said and done, we're left with very little to speculate on what Jesus might have been writing (if he even could write - most modern scholarship suggests otherwise) or whether this story ever happened in the first place. And in the end, I find that answer far more important than any speculation on what may or may not have been written.
(1) B. M. Metzger's excellent A Textual Commentary on the Greek New Testament is the best place to begin searching for a textual approach to the New Testament based on actual evidence presented by differing pericope. I couldn't recommend it more.