Non-touchscreen based smartphones are designed to be used one handed. They are however more limited with regards to interacting with software as there is no stylus or touchscreen. Touchscreen based devices are much more powerful with regards to software operation, but are more difficult to use one handed.
Pocket PC Thoughts has an article that may be the solution to this problem. Innovisoft has released VirtuaMouse One-Hand software for Windows Mobile 2003 and newer devices that have a D-Pad. (The 5-way controller that is found on most touchscreen based devices)
If this sounds interesting, click HERE for the Pocket PC Thoughts article.