Within my first half hour I had accidentally unplugged it 5 or 6 times, just by moving it around in my lap. Pick up the cord and plug it back in each time, it starts to get a little annoying.
I've gotten better with it now, but I will still do what this video shows, where it doesn't completely unplug. You don't always notice until you get a low battery warning when you thought it was plugged in the whole time :(
I had two of my old chargers that broke. I've also never had an issue with the new charger disconnecting during that last month with my new Macbook Air.
I do agree it's annoying, my solution is to use large kitchen tray so that the whole computer and it's surrounding is protected from my knees. There are also easy to buy lap-desks (ikea has something), beanie bag underneath and flat plastic top.