Here's what's in my main travel bag (lots of people ask me, surprisingly):
First off, I use an O'gio City Corp bag as my primary travel bag. I love O'gio bags because they've got lots of internal and external pockets and they look great embroidered with company logos. Additionally, the hardware on the bags is first-rate and does not easily break or wear-out. The final thing I like about the bag is that it has a generously-sized laptop packet and a holder on the outside for a cell phone that I use for my Blackberry 850; the holder has a cool pull string that literally ejects the Blackberry when I need to read it.
My primary computer is an IBM T40. The T40 is pretty thin and light with a large screen and full-size keyboard. Like most IBM laptops, the thing is bomb-proof; it takes drops and bumps like a champ. I really enjoy the Centrino chipset; I connect wirelessly to the Internet in airports, at Starbucks, at Borders, basically anywhere I can find a connection when I'm traveling. Inside my bag I've got a power adapter, a basic 5 foot length of phone cable, and a 5 foot length of CAT5 cable; all of these items necessary to power and connect the computer respectively.
I have an older generation 30gb iPod that has about 20GB of music on it. To support the iPod, I carry a power adapter and the USB/Firewire cable (the T40 does not have a USB port); it's nice to be able to use the iPod as a portable hard drive if I need to. The only other things for my iPod that I carry are the original Apple carrying case and the headphones. For extended trips, I do have the inMotion speaker setup, but it doesn't live in my bag.
For quick portable storage, I have a SanDisk 128MB thumb drive. Jump drives are really useful when you have to give presentations or really need to quickly share files with someone and your computers are not networked together.
My current digital camera of choice is the Canon PowerShot A80. The A80 has a lot of picture options that can be controlled while taking pictures; it's point and shoot if you need it to be and semi-professional when you need it to be. The 4MP pictures it takes are awesome and more than what I need for my personal or business use, and the storage is infinitely expandable with Compact Flash cards; currently my camera has a 256MB card in it.
For a cell phone I use a Motorola i730 with Nextel service. I know lots of people that use Direct Connect and the free incoming plans are certainly a money saver. I carry the phone and a rapid rate charger. I am looking forward to the forthcoming i930 that is supposed to run Windows Mobile and have a 1MP camera.
That pretty much wraps up all the electronics and crap in my bag. Most trips see at least a book or two and several magazines along with my Tumi padfolio and any paperwork and travel documentation I may need.
All of this does fit under the seat in front of me and has never needed to be put into the overhead bin.