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I use mine mainly on hard surfaces. Desk at home, tables at the library, tables at coffee shops, desk at work, desks in school, and the dinner table.

The only time I use it on my lap is when I get home from work/school, I have my laptop in my bag, and I'm too lazy to set up my desk. I'll just plop down on the couch, turn on the TV, and start zippin' around the internet on my computer.
 
-Class
-Library
-basically anywhere on campus, really
-Living room sofa

I might occasionally use it at a coffee shop or something, but I find that a bit unnecessary unless you've taken to always drinking coffee by yourself.

I never use my Macbook at my desk, though; I've got a desktop for that.
 
I primarily use my MBP either on my desk or on my lap when I'm in a lecture hall for class. Sometimes I use it in bed.
 
- Kitchen table (even with breakfast!)
- Couch at granny's. (Awesome place with tethered internet!)
- Bed
- Toilet
- Local (non-computer) games centre/café, for when I'm bored playing Magic: The Gathering for a while.
- Café's and other places when I'm alone.
- At buses, waiting for the bus to arrive at the bus stop goes so much faster with tethered internet and my lappy!
- At bus stops, waiting for a bus to arrive. (Countryside or small cities, no metropolian bus stops of course!)
- When walking: no need for an iPod if you have iTunes, InsomniaX and a good set of headphones!
- All places you can imagine to bring an iPad with you, or - like previous example - bring an iPod with you. :D
- Car while long trips and not driving
- When playing at the MBP's at the Apple Store to compare them to my Mac

So a better question would be: where don't I bring my Mac with me?
 
I use mine on the desk most of the time because that is where the external screen + mouse is.
On battery I also often just use it in the garden in a hammock or in my room in a hammock or in the winter garden.
 
Nice thread :D

desk, lap, sofa - at work, at home, and in cafe's

I've used it on trains, at bus stops and in public transport, but not so often. I can't afford business class flights, so almost no use on planes. I agree that for the toilet, the perfect device is the iPad.
 
Mostly on my lap as sit on my bed due to absence of a desk at the moment, otherwise on the table
 
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