I only ever have my laptop out during rehearsal when I'm doing linenotes - I find it much easier to type them than to write them down.
I do so, as well. My typing is much faster (and more legible!) than my writing. And since I'm going to e-mail line note to the actors after rehearsal, anyway, *shrug*.
I also do scene shift charts on the computer, during rehearsal. When the director suddenly adds a prop whose location has to be charted, or decides he wants another extra in the scene, it's far easier and more accurate to make the changes on the file.
During rehearsal I usually have the following files open on my computer: line notes, shift plots, actor/scene breakdown, rehearsal report, and depending on the director, rehearsal schedule.
Of course I have all these in paper in my book, as well. I don't think I'll ever go totally digital, as in book on computer, or IMing other departments in rehearsal, but my laptop is a tool that I definitely feel makes me a better SM. It makes my paperwork faster, more accurate and more efficient, that's for sure.