... I wanted a turtle *cough*slave*cough* digging while I set up other things.
But I'm afraid of ruining the landscape.
The basic problem is that the turtle will need to place more blocks than it digs. So, you need to replenish its stock of blocks. It's an interesting challenge.
Lets break it down.
1. Place an ender chest in slot 1
2. Set something up to empty another identical ender chest but keep two specific slots full of cobble. I'd use Steve's Factory Manager, but you could just as easily use another turtle that sits in between an ender chest and an ME2 Interface.
3. Place the Ender Chest above the turtle
4. Grab a stack of cobble into slots 2 and 3
5. Break the Enber Chest back into slot 1
6. Dig down, move down
7. Place a piece of cobble from slot 2 above the turtle
8. Repeat 6,7 until bedrock is hit
9. Dig forward, move forward
10. Turn around, place a piece of cobble in front
11. Dig up, move up
12. Place a piece of cobble from slot 3 below the turtle
13. Repeat 11,12 until target height is hit, e.g 64
14. Turn around, dig up just in case
15. Place the Ender Chest above the turtle
16. Dump everything into the ender chest
17. GOTO 4
This just quarries in a straight line. Quarrying an area is a little more tricky, and is subject to interruption on server re-starts or chunk unload, so I'd set up a phalanx of identical turtles that all just mine straight forwards. Chunk loading could be done by a co-ordinator turtle that follows the fleet, but keeping them all together might be difficult. You'd need some kind of GPS really, and then you're into range limit problems.