I used 1x3 box pine instead of 2x4's for the bits that things sit on, and everything fits perfectly.
Originally I was planning on a 3.6m long x 0.6m deep X 1.8m high shelf. At the lumberyard I found out that 2x4's came in 6m lengths as standard, so I adjusted this to 1.5m high instead, which worked out fine as I think 1.8m would have been a bit tall to get heavy things on top of. Luckily for me my power points and light switches were just under this, which I didn't think about at the time!
Originally I was going to try and cut grooves for the 'rungs' of the ladder frames to slot into, hence the insets at the bottoms of the legs. I think that the glue-and-screw construction will be plenty strong enough, though.
This took me about 4 hours to put together, and cost about NZD$170 all up, which is about USD$120.
I've got the bug. Making things is awesome.