Last time I farmed obsidian, it was before place-able string... Redstone was the only item you could use. Now I just buy it, or avoid using it.
I still use redstone, although I have not checked recently to see if my machine design has (finally) been broken in 1.7, mostly because I have been in one of my outposts mainly and when I need obsidian, I just go looking for an above ground lava pit to water.. xD
I changed it to left click only a while back for that reason. So, unless your machine requires punching blocks with string in hand, your good now.