After exploring few options for iPad grid structure, I designed this grid. Thought may be helpful for other designers. We all are aware of grids, but just for the reference you can download it here.

This grid

1. Designed for 1024 X 768
2. Works well in both vertical and horizontal orientation
3. Unit size is designed based on minimum tap-able area which is 44 X 44 px
4. Proposed column structure for horizontal orientation (4 columns) and vertical orientation (3 columns)
5. Feel free to modify the gutter but keep in mind the minimum tap-able area and the porportion of negitive space in layout. (This design has minimum possible gutter)

Do correct me if I have missed on any detail.
I am now working on creating (16:9, 16:10) more grids.

Also folks you can download iPad GUI here, thanks to Teenhan + Lax.