I carry a laptop to and from work and school and gym, a smaller Sony Vio, but sometimes I also carry my old HP desktop replacement machine...
I carry them inside a laptop case, and that goes inside my Timbuk2 messenger bag. I highly recommend this method of carry. It is uber comfortable and convienent. Also, no worries about leaning, as it has a chest strap that goes around and keeps the bag in place on the small of my back.
For trips where I'm packing gear and traveling, I do everything in my power to leave the laptop at home... I'm trying to get away from it, not take it with me! For 90% of the time in those cases, my PDA was enough... my new PDA is a smartphone, so I can stay 'connected', but I often use the 'poor cell coverage' excuse.
For the one time on a business trip where I
had to take a laptop, I packed it in my laptop bag and that inside my Tourmaster tailbag strapped onto my bike. For the trip home I had it shipped home ahead of me.
Seriously, I highly recommend Timbuk2 bags for carrying stuff
http://www.timbuk2.com/tb2/retail/catalog.htm