I think they chose the [] for arrays and {} for dictionaries as that is the style that seems to be most popular right now. Python for instance uses this.
When adding features to something, I tend to be a big fan of "when in Rome"[1]. If these were extensions to Python (or Ruby or JavaScript) then I would totally be for using '[' with arrays. As a C guy though, I would rather have seen them use '{' for NSArray and '[' for NSDictionary. Regardless, I'm glad to see this stuff added.
It's already custom in objc to use {} for dicts and [] for arrays when printing them out, unless I'm misremembering. It also matches the use of those characters in JSON, so it seems natural to me.
[1] http://en.wiktionary.org/wiki/when_in_Rome,_do_as_the_Romans...