Default keyboard shortcut would be F6 to set focus on the Location bar. Ctrl+L clears the location bar and then sets focus on it. Both shortcuts can be changed in the Settings menu -> Configure Shortcuts.
OS: Kubuntu 6.06 LTS (Dapper Drake) | KDE 3.5.3
Computer Specs: AMD Sempron 2200 1.5Ghz | VIA KM266 Pro 8235 chipset | nVidia GeForce MX 4000 128MB DDR-RAM 32-bit AGP 8x