I usually carry the ipad around with me, it easily fits into my purse. I'm sure there's a man-bag that could work for you. Having it with me means lots of options if I run into downtime, or if I have to entertain bored kids. It has saved me when I needed directions (mcd's has free wifi!) and I have used it shopping (lists, preloading shop websites). I found it much easier to tote while traveling than a laptop. Airplane travel is especially suited.
I don't have an iphone however, which is much more portable. The times I leave the iPad home is when I think a) I don't want to carry a purse or need the room in my purse for other things, b) I'd have to leave my purse in an unsecure area, c) weather is bad (iphone can hide easier in waterproof coat).
It would be even more indispensible if I had 3g or some kind of tethering.