I don't have a netbook, but I have an underpowered and almost dead laptop so it's almost the same. There's a few things you'll have to get used to, but the ultimate recommendation depends on you're specific needs.
- Keyboard--This is the most important part of the netbook. If your wrists are cocked to the side too far it becomes really uncomfortable really fast. If you can, go to the store and spend a little time using the keyboard. Open a text editor and just start spitting out
hello world style programs until you have an opinion on the keyboard (it'll work or it won't). Don't get cheap on the keyboard. Everything else is secondary.
- Clamshells--Everyone complains about the screen real estate, but forgets about space in public transportation or airplanes. When you have a big screen, the clamshell can't open as far which in turn makes it difficult to see the screen.
- Screen real estate--Pixels count more than inches. Don't underestimate the amount of screen taken up by your operating system--the launcher and status bars at the top and bottom of Ubuntu desktop, the task bar at the bottom of Windows 7, etc. all take up an appreciable amount of screen real estate. Hide them if you can.
- Clear text--With smaller screen real estate (pixels here), you have fewer pixels to represent your characters. Make sure that the OS you use has good code editing fonts that don't strain your eyes. Most modern operating systems from Mac to Windows to Linux have good anti-aliasing support, and you'll need it. You might need to tweak some settings, and you might have to download a good programming font (better than Courier New), but you don't want to introduce eye fatigue. Also, use a larger font size than 10pt. You may not be able to see as many lines on a screen, but the text will be clearer. That can make the difference between 10 minutes before your eyes get tired and several hours.
- Battery life--This is probably the second most important aspect of a netbook. My old laptop is abysmal at this, and if I use it for more than 40 minutes untethered (just idling) I run out. If you intend to use this on a train or other transportation with no outlets, get the best battery life you can. You won't get the rated life, particularly if you are compiling, but it can mean the difference between 20 minutes before you have to shut down and being able to use the machine for the whole trip.
Hopefully it doesn't need to be said, but you don't buy a netbook for processing power. If you are working with complicated languages to compile like C++, you'll probably spend more time waiting for the compiler to get done and less time actually working. That said, my ancient of days laptop only has a 1-core 1Ghz processor, and I get along OK.
Programming stacks do take up a fair amount of RAM. I would recommend no less than 2GB of RAM in your machine--even with Ubuntu.
Note about IDEs: You will have better use of your screen with a high powered text editor like Vim or Emacs, but some IDEs provide a decent balance. The guys at JetBrains have a decent enough interface, and the niceties built into the IDEs really help you think about the problem you are trying to solve rather than the language you are trying to use. The support panels can all be collapsed out of the way so you can focus on editing text, but still provides the refactoring support, programming by intention, type-ahead, etc. You will be making use of hiding the panels more often so make sure you keep that in mind.