I bought an 11 inch netbook to put behind my DM screen for running encounters. Using the 4eD&DCM tracker, I can handle much more complex encounters than I previously could. Also, thanks to the Adventure tools and Character builder, I have access to everyone's characters and all the monsters neede for a quick combat, if I need to put something together.
I no longer bring any books with me. There's a PH on hand if needed, but I can usually look up whatever we need on the compendium faster than flipping through the book.
I used to be against laptop GMing, but it's really benefitted me so much that I'll never run a game without it.
On the flip side, I'm not keen on players using laptops, mostly due to space allowance and distractions when their not involved.