If you do go for reusable nappies, I still strongly recommend disposables for the first 2-3 weeks - meconium's impossible to get off cloth nappies and it goes EVERYWHERE, whether or not you use a liner! And you will probably still need disposables as back up, e.g. if you're going out for the day/got a long journey, etc, unless you're really die-hard. DD couldn't go more than 2hrs in cloth, so was in disposables at night. I still had to change her twice a night. She was a heavywetter though.
Muslins are obviously necessary - can I also suggest a pile of old towels?! Useful for nappy-free time, for covering the sofa, as a liner for the changing mat... DD had/has severe reflux so I'm probably more extreme than most but we had towels everywhere for the first 18 months. Now she just clears up her own puke :-)
Has anyone said nappy bags?? Even if you use reusables, trust me, you don't want to put poo cotton wool straight in the bin.
LAUNDRY DETERGENT - more than you ever thought you'd need in a year. Not just the baby's clothes, but the muslins, the bedding, your bedding, your clothes... you get the picture! So much easier to stock up on the supersize ones now, rather than try to drag a buggy round a supermarket because you've run out.
New memory card for your camera. Ours had space for just 7 pictures when DD was born. Didn't even think of checking beforehand...
plus all the obvious from above...