"In my former book, Theophilus, I wrote about all that Jesus began to do and to teach"
Who was he?
Is it also possible that, since Theophilus means, "friend of God", as Richard states, that Acts is written to all believers?
Theophilus was the person addressed in both the Gospel of Luke and in the Acts of the Apostles (ie "Luke" and "Acts"). The identity of Theophilus is unknown. Per Wikipedia there are multiple possible options:
He was a specific individual
Theories say that it could have possibly referred to a Jewish priest, a Roman official, or a lawyer friend of Luke's.
They were a group of people
In this case, it would be addressed to a generic group of academics (since it was an honorary title, it would apply to anyone reading it--academics of the day).
It was a generic person
This idea is that Luke was writing to someone unknown to him, or just writing to a generic person so that the letter could be used for anyone and everyone.
Unfortunately, we don't really know.
What we do know, however, is that Theophilus, in the original Greek, means "friend of God", "loved by God", or "loving God". This was a common name back then, but it was also an honorary title used among the academics of the day.
Wikipedia has much more info on the subject.