I found myself needing about 15,000 obsidian to build a suitable mage tower for my Thaumcraft area, so instead of spending about 24 hours straight with a diamond drill, it was time to build an obsidian generator.
Fortunately, IC2 lets you bring water into the nether in the form of ice, and with the addition of RP pumps, it's pretty easy. All you need is a pump, block breaker, repeater and pulse former. This uses the appearance of a block of obsidian to trigger the block breaker, rather than a timer, so it goes as fast as the obsidian appears - a little faster than one block/sec (same would work for a cobble gen).
I have 15 thermopiles running the pump, but sadly, even that isn't enough to keep it running continuously. Starting with a full battery box will at least get you almost a full single chest before it quits, though. If you try to build this, be sure to break the ice blocks you place next to the thermopiles manually instead of letting them melt. Thermopiles will eat the ice blocks occasionally.
This is after one full single chest. Gives an idea of the range of the RP pump. This lava ocean is about 15 blocks deep at this point, and the intake grate is right at the bottom.