Big, powerful laptops have their place -- but they're completely useless for taking on the road. They're often extremely heavy and, despite manufacturers' claims, have very limited battery life.
Ultra-portable laptops are far better for those on the move. Most ultra-portables tip the scales at around 2kg, but some models are even lighter. There tends to be a correlation between the size of a laptop's screen and its overall weight, so the trick in choosing the right model is to decide how large a screen you need in order to stay productive.
Ultra-portable laptops are typically less powerful than their larger brethren. Slow CPUs are chosen because their faster counterparts can drain a laptop's battery in a matter of minutes. As a general rule, the slower the CPU, the longer the battery will last. Some laptops have hot-swappable batteries. An internal power cell keeps the laptop running while you swap batteries, so you can keep working without losing your data.
You shouldn't worry too much about slow processors impeding your productivity. Even a 1GHz processor is capable of running everyday office productivity applications and basic image editing tasks with relative ease.
Your other major consideration should be connectivity. The more ways your laptop can interact with the outside world, the better. Most laptops use Intel Centrino technology -- a tech standard that states a laptop should use a particular combination of CPU, mainboard chipset and wireless network interface.
All Centrino laptops are compatible with 802.11b/g Wi-Fi networks, and newer models now incorporate the 802.11a standard, so no matter which type of Wi-Fi network your local coffee shop uses, you'll be able to get online. Keep an eye out for laptops that also incorporate Bluetooth -- these will let you synchronise data on your mobile phone or handheld organiser with ease.