Sketchup is a great tool for modeling all sorts of shapes and objects. One of its biggest drawbacks is that it has a hard time modeling organic shapes with NURB attributes but luckily it doesn't have to stay that way any longer thanks to the Soap Skin & Bubble plugin.
Josef Leibinger's Soap Skin & Bubble plugin for Sketchup is one of the most exciting things to happen to Sketchup in quite some time. The free plugin gives modelers the ability to easily create a multi-faceted face inside of a closed group of three or more lines. Its as simple as selecting the lines, start up the plugin, tell it how many faces you want the new face to have, and then sit back to watch it work its magic. The plugin also lets you inflate or deflate the new face after its been created.
As a word of warning, if you use a very large number of polygons to create your curved face, the plugin may crash your model depending on your machine. It's rare but if you keep the polygon count low you shouldn't have any problems. Overall, its a stable plugin that is a blast to use on more organic shaped models. be sure to follow the steps on his download page to install the Soap Skin & Bubble plugin properly.
Soap Skin & Bubble Plugin
UPDATE: Soap Skin & Bubble Plugin can now be downloaded directly from Sketchup's Extension Warehouse or installed via Sketchup's Extension Manager plugin.
Soap Skin & Bubble Plugin via Sketchup Extension Warehouse