I am designing an iPad app, so I could really use a grid template. I could probably just work with the 960 grid system; however, I'd love to find a solution that is tailor-made for the purpose (that is, takes into account such things as the iPad's resolution and suggested (by Xcode's Interface Builder) screen margins). Any suggestions? How do you approach iPad interface design?
There aren't any grid systems for native iOS development. You have to do your own grids in photoshop.
I am mostly not going to use default iOS interface elements
The most common way of doing your own components is to extend iOS components with your own skins and behaviors.
If you go for full custom components then you extend the UIView and build your elements on top of that, but IMHO for most components it is a costly approach, i.e., lot of development work that you need to do yourself.
Also from a ux perpective it is a good practice to follow the OS UI Guidelines so it is good to use or extend native components.
I'm not saying you have to do exactly what the guidelines say or to use native components for everything, but it is a good place to start.
Guidelines are recommendations that were tested (at least we hope so) and that users get to know and get accustomed to, they are not unbreakable rules.
You can find a list of iPad stencils here: http://emilychang.com/2010/03/ipad-templates-and-stencils/
most are in PSD format bit I did not used them myself.
I've been experimenting with grid layouts for the iPad/iPhone and I've started a collection of PSD templates on GitHub. They're intended for web design but they could also work for designing native app comps.