My opinion and suggestions

Great mod which made me minecraft addict again!

I can't detect diamonds with turtle, because can't put diamond "ore" in inventory.
Make detection based on item id instead selected item slot.

Wireless thing is quite wierd too. It isn't so logical that wireless works only front,back,right and left and not circle around target.
Make wireless work circle around turtle like real wireless.

GPS would be also much better without those little "gps towers". Those "towers" limit range where turtle can move and also limits ways to use turtle.
Make GPS work globally everywhere without towers.

There is also lot different items that don't really add any features but just makes mod more complex.
Few example what is difference with wireless turtle and turtle?
Floppy drive makes things more complex when you need to set it next to turtle everytime you want read custom code from disket.
Maybe let computer and turtle take floppy inside instead external floppy drive?
Focus on mod main point programming and make api better instead adding more complex and useless items!

#1 Read #3. It'd be hard to map item id -> block placed, in addition.
#2, I'm not sure what you're saying. When opening? That's because that's where the modem (peripheral) is located. You should be able to send stuff to any other modem in range.
#3 This won't happen. CC is 'computers in minecraft', from what I can understand. This means no 'magic' (such as using Notch's coordinates, or item IDs.)
#4 These are perfectly reasonable. A turtle has no wireless, but a wireless turtle has wireless.
Floppy drives are fine, in my opinion. If you want, you can place a floppy drive in the ground and place turtles on top of it. They will (all) start with the startup script in the disk (if one is in the floppy). This is very useful.
Instead of saying 'make better APIs', perhaps you could give us some examples?