Hi Karen:
Yes, you would be able to carry your laptop in a neoprene sleeve in your carryon. I've done so on a few trips. (As other posters have said, you'd have to take it out and put it back in when going thru security but that's not a huge deal.) Just make sure it's packed securely so it doesn't move around/get jostled too much.
Something minor to consider, too, is whether you want to keep it in there once it's on the plane. If the overhead bin above you is full, will you be able to keep it in a bin where you can keep a reasonable eye on it? Also, sometimes other people move things around in overhead bins so it might get a bit jostled.
Also, every once in a while, there's not enough overhead bin space and flight attendants have to start gate checking bags. So either make sure you get on the plane early enough to try to get an overhead bin, or leave enough room in your messenger bag to put the laptop in there in case you have to.