The 3DMouse code was rolled into Blender's main branch some time ago.
All current version of Blender 2.6x have it built in. Still much refining left to do.
Currently, all motion is relative to a fixed point, usually the last selected object that you 'fit' to the viewport.
Holding shift allows you to pan along the three axis, which also moves the center of rotation. Without shift, you are limited to rotation and forward/back zoom.
Free flight without shift is in the works.
Remember, there is essentially one person writing most of this code... for free... so be patient. The rest of us are all beta testers telling him what he did wrong, or what is still broken.
Additionally, if you use the OpenSource Spacenav
driver, you can use the, so called 'obsolete' spacemice, including the serial models.
So far, under Linux, the following are known-working with Blender/Spacenav...
All of the USB devices -should- work, though the LCD displays are not yet supported. This is mostly because none of the spacenav developers have a pilot to troubleshoot with.