Laptops come in all sizes. This is true for ones that are powered by Linux, too. When you need power and if you're not on a budget, check out Emperor Linux. The company sells what it calls "professional grade Linux laptops." They're powerful, but pricey: between $1,400 and $5,400 (USD). You'll get a brand-name laptop -- like Panasonic, Sony, or Lenovo -- and you can choose the Linux distribution that you want.
Slightly cheaper, but no less useful, are the Linux laptops from Dell, System 76, and ZaReason. Even when you customize the systems, the prices can be reasonable. The notebooks from these companies run Ubuntu, but with ZaReason you can specify the variant of Ubuntu that you want -- Ubuntu, Kubuntu, or Edubuntu.
If you want to go smaller, you can pick up the Eee PC, and the Acer Aspire One.
You can find a long list of Linux-powered laptop resellers here.