That's interesting, I didn't know that. So a full bishop is normally the one to conduct services? I suppose I had always assumed something very different, that priests handled the day-to-day operations of a church, including preaching, and that bishops were more in an administrative role. I may want to check on one thing with you to make sure I'm thinking of the right thing. By "preaching" do you basically mean saying the homily? My understanding is that a homily is akin to a sermon.
If a bishop is present, he is the one who serves the Divine Liturgy, unless he chooses to delegate someone else to do it. The other services he will usually delegate to a priest. I know one bishop who always chants the Vigil on kliros every weekend with the choir, while the dean of the cathedral is the principal celebrant.
In Orthodox ecclesiology, a priest is only the bishop's representative. He can only operate within the duties delegated specifically to him.
While bishops do have administrative responsibilities, according to what they are charged with at their consecration, it is the *spiritual* oversight of their diocese that they are responsible. Good bishops tend to delegate most administrative responsibilities (while keeping a close watch), and concentrate on prayer, the services, and direct ministry to the people of their dioceses.
In modern (i.e. that of the last 1000 years or so) Orthodox practice, dioceses are fairly large and have many parishes. In our diocese, it is an accomplishment for the bishop to make it to each parish in the diocese once a year. The clergy go to gatherings with the bishop at least twice a year, and laity are also welcome to come to these assemblies.
So, priests do handle the day-to-day operations of a parish, including preaching, in most cases. Historically it has been fairly common for priests not to preach, though. Preaching would be done by those especially delegated by the bishop to do so, and sometimes the bishop would distribute sermons to be read.
I would also point out that preaching is not really appointed to be a part of the Divine Liturgy. Although it is usually done there today here in America, that's not where it belongs, according to the service books. The Divine Liturgy has its own life, and transcends time, and preaching interrupts that. The traditional places for sermons are at Matins. But, because attendance is overwhelmingly best at Liturgy, sermons have moved in recent times into the Divine Liturgy, after the Gospel, or preferably, after the conclusion of the Divine Liturgy.
Yes, homily, sermon, preaching are all the same things. I would also point out that there is another strong tradition in the Orthodox Church that has been lost in the last century or so, and that is the reading of liturgical sermons. It is usually hard for most priests to improve upon the words of the Fathers. I always deeply respect priests who draw heavily from patristic sermons. There are sermons and Scriptural explanations and lives of saints appointed to be read in Church every day of the year. This practice is used too little these days.
And of course, even without preaching or the reading of liturgical sermons, the texts of the services themselves are so chock-full of theology that one can go on overload just listening attentively to the prayers, joining in with one's heart (or lips, if you happen to know the particular hymn being chanted.)