Messenger backs just flop around to much. OK for papers and other light items, but for a lap top will flop to much. A back pack would be better for a Back pack attached to you body at the waist and BOTH shoulders unlike just one Shoulder for a Messenger bag. Every time I have used something like a Messenger Bag over my Shoulder, the bag shifted all over my body as I biked. Sometime up or down depending on which way I was turning, sometime when I hit a pothole, sometime for no apparent reason (and sometime when whatever in the bag shifts, and this is worse if the item in the bag is heavier than a few papers). The big problem with a back pack is that a backpack prevent sweat in the middle of your back from perspiring into the atmosphere, thus the better item is to get a padded bag for laptop and then put the bag (with the Lap top) in one of a pair of Rear panniers (Or Saddlebags). Panniers just do NOT flop around for they are held in place by the Rear Rack. I use Cannondale Panniers and a Tubus Rack. I purchased the Tubus rack from White Wheels in New England, but most bags and racks will work provided the pannier are big enough for the laptop.
If you do NOT want to spend $100 for a rack, your local bike store should have Rear racks. Most bike stores have racks. Avoid solid Aluminum racks, they are weak and tend to break after a few months. The best racks use hollow steel rods, Second best are Hollow Aluminum Rods, the Third are Solid Steel racks. The Tubus use Solid Steel Rods and in the two four years I have been using them I have NEVER had any problem.
Now my Cannondale Panniers (or Saddlebags) are older then my rack. I have abused them using them constantly for the last five years. They are big enough for a laptop in a protective bag.
Also Avoid bag on TOP of the rack, they tend NOT to attached to the rack as well as Side panniers, thus I would NOT trust a Laptop in one. I would NOT put the Laptop in a FRONT panniers. I have used Front Panniers and they work well for LIGHT ITEMS (Such as Clothes) but a heavy item like a Laptop tend to change the handling of the Bike (Unlike Rear panniers where the change do to the weight is Minimal).
For Tubus Racks (Peter White also sells high end Pannier bag):
http://www.peterwhitecycles.com/tubus_racks.aspCannondale panniers (Saddlebag):
http://fr.cannondale.com/clothing/04/ww/model-BPN340.html#http://fr.cannondale.com/clothing/04/ww/index_gear.htmlOne last comment, in addition to Panniers I use a rigid handlebar bag for things like hats and gloves and other small items I think I would need on the Trip (NOT A LAP TOP). I would recommend such a bag for such little things such as the things you carry in your pockets but may fall out when you are biking. I also would recommend a Seat bag for hauling some tools to fix your bike if something would go wrong, along with a inner-tire patch kit. Together all three sets of bags work together. Your tools are easy to get to, if you need gloves or a hat they are in front of you (or if you are to hot you have a convenient place to put your hat and gloves), and anything heavy can be in the Panniers.