Same here, the only time I had it weighed was Qantas, BNE-ADL. They told me I'd have to check it and I refused because it was my laptop, etc. They then offered me a small plastic bag which fit laptop, wallet and I checked the back pack.

I didn't weigh it this time but I'd guestimate about 15 lbs: smaller laptop, digital camera (and both of their accessories) and two days' clothes because my larger backpack was being checked through to Prague whereas I was staying in London for 24 hours.

I tend to overpack it with books.