There are netbooks and netbooks - I own both an MSI Wind U-100 (now 3 years old) and an Asus Eee 900 (two). The first one got an extra stick of RAM, and I swapped their HDD. Moreover, I installed Linux on both instead of XP Home (MSI) or Win7 Starter (Asus) - performance got much better with a bloat-less OS, and desktop compositing does help on managing the screen's real estate through some plugins.
The Eee is now used as a small web browser and email client - its battery soon lost its capacity, so it's not mobile any more and prety much nothing more than a smart terminal.
The MSI though, is still going strong with more than 2 hours of battery life (it's the original 3-cells version) in light browsing mode, its latest BIOS update and a few tweaks solved all its Linux compatibility problems, it's fast enough for mobile use and I can overclock it by 20% when on AC - enough to watch many MPEG-4 720p videos on Youtube. With a USB keyboard and mouse, a DVD burner and a HD screen plugged in, it's a solid office PC; on the move, with the smartphone plugged in as a 3G modem, it's the perfect mobile office. I have rather large hands, so typing for a long time can be a bit straining after a while, but for most uses the keyboard is more than good enough - at least compared to the tablets + Bluetooth keyboards I tried.
I wouldn't say no to a better screen, a beefier processor, a small SSD (for battery and hibernate/resume speed) instead of the HDD and one more USB port, but I'd rather keep its current power level, form factor and price - things that an ultrabook. doesn't have.