Have a look under Edit->Insert Text Macro->Objective C->Bracket Expression. This will insert a matched pair of brackets (or if you have something highlighted, put brackets around it). You could bind this to [ as a keyboard shortcut.
You can follow the instructions here to create one for parentheses. I tried but couldn't make it work.
http://cocoawithlove.com/2008/06/hidden-xcode-build-debug-and-template.html#textmacros
Personally, I drag the classes folder over to Textmate and edit there. I switch back to Xcode when I need to type in some long method name, or to build. I've been building a library of snippets for some of the common things I do in Textmate to make life easier there.