So playing around with my iPad, I went ahead and shelled out the $9 for Sketchbook Pro from Autocad, since it is a very full featured sketching app. One nice thing is that it supports a simple from of Photoshop layers. So I have been playing around with it to see how well it can be used for making game maps while I am waiting for the train, or sitting through a boring meeting. All you need to do is create your grid in a program like Illustrator and save it out as a PNG, with everything other then the lines set to be transparent. The images should be 1024x768. Here are a couple I made using illustrator:
Old-school cyan lines:
Just download them, and then transfer them into the iPad Photo Gallery somehow (I just emailed them to myself, and then opened the email on the ipad and saved it out there. But there is probably a smarter way to do it.)
You then open up Sketchbook Pro, and go into the layers flyout:
and then select the 'flower+' icon at the bottom to import the grid image from your photo library. (since they are transparent PNGs, the app shows them as solid black thumbnails, but they work fine.)
You then can create a new layer and put it behind the grid image layer. I also lowered the opacity of the grid layer to make it less intense. you then sketch on the layer(s) behind the layer with the grid image. the app supports upto 3 layers, so you can have the Grid on top, your background texture at the bottom, and then sketch out the dungeon or terrain features on the middle layer. i will try and post anything I make that comes out decent.
On may 28th, Autocad published an update to Sketchbook pro making the transparent PNGs less necessary - they added blend options to the layers in the app. So just like Photoshop you can take a scan of your favorite grid and place it in the top layer spot. and set its blend to Multiply. But these PNGs I created can be used, and then blend the layers differently to get other effects.