I am not sure how to put it, but...

It's not going to happen. :-(

Lighter and mobile laptops come at a premium. In fact, I know this because I do know of one product that I currently own that fits your requirement down to the last character (except for the <$500 price range).

I bought a Sony VAIO TX 650 laptop in Jan 2006 and ever since then I have had no troubles whatsoever mechanically with it. Yes, it's a bit pricey in the $2000 to $3000 range, but in my opinion something that expensive from a reputable electronics manufacturer is worth my hard-earned money for.

I have used my laptop on many flights in the past several months and I can tell you that no matter how small the seat pitch (US Airways and Continenal come to mind) was, I could use it with ease, even if the passenger in front of me is reclined all the way. I happen to be very overweight at 5ft8in and 295lbs, but the laptop can fit on any tray table perfectly and I still have room to type without effort. It has a battery that has a life of about 6 hours at full charge. In addition, the current models have Sprint EVDO modems on-board (however in my case, they featured Cingular EDGE modems) so speed on mobile networks so access speed is almost never an issue.

In regards to using Bluetooth on the plane, legally it's prohibited right alongside cell phones and wifi, but IMHO it should not interfere with the plane's navigation aids since the effective range of Bluetooth devices are less than 10m from the device.