I'll take a stab at it, although I must confess that I don't like the new turning undead rules in 3rd edition. I feel that they're too powerful. Here goes:
Clerics can turn undead for a number of rounds/day equal to:
(5 + Charisma Modifier) * class level
Each turning the cleric can affect any number in line of sight. An undead creature is turned/rebuked if
(d20 + Cleric's Charisma Modifier + Cleric's class level) >=
(d20 + Undead's Wisdom Modifier + Undead's Hit Dice + 2 ) +
(1 per 5 feet between Cleric and Undead creature) +
4, if undead is attacked by cleric or his allies
Clerics may turn undead only once per encounter.
I chose this radius method to reduce the number of rolls on the player's part and because I thought that it would be interesting. Also, adding a roll for the DM allows for more surprises in the realm of the undead. I felt that the +2 modifier in favor of the undead helps to even out the adjustments for 1st level clerics against 1-HD undead. After all, a 1st level cleric facing two medium skeletons is not supposed to almost always win, due to a good charisma. Comments?