I keep my laptop open during rehearsals. During the typical rehearsal, I have a blank rehearsal report, blank rehearsal report, prop preset paperwork, and scene/character breakdown open almost all the time. Most of the production heads and staff at the theatre know that the most unobtrusive way to get a hold of me during rehearsals is to shoot me an email (much less distracting to everyone else in the rehearsal hall than having to answer a phone - even with the ringer off) that I usually can get a response off to in the next couple of minutes.
As far as programs, I pretty much stick to Microsoft Office programs - mostly Word and Excel. I have tinkered around with OpenOffice as well and have found it to be a great alternative and well worth it for people who are not willing to drop the money that Microsoft charges.
Having a laptop open in rehearsals is great for those shows where there is no dramaturg in rehearsals, too. A director wants to know how much a British pound was worth during the 1950's and you can look awful good getting them the answer in 30 seconds with a computer, internet connection, and Google.
I don't know if it will ever replace my binder, though. Having the most up to date paperwork on there is great and I do have most everything that is in my binder also on my laptop. I bring it to most meetings as well and can edit schedules or other paperwork in meetings as things are discussed. I actually have never used it for line notes. Perhaps because that is something that I usually leave for my ASM.
I've never had a director, actor, or anyone say that my laptop is a distraction in rehearsal. And once I get into tech, everyone and their assistants seems to have their laptop or tablet PC open to the point where I sometimes have to remind people to shut their lids to get a decent blackout in the theatre.