A number of interesting VR applications support the spaceball under linux. While the available software may not be suitable yet for end-users, it shows a great potential for academic projects, and it may be an escape route from vendor lock-in for institutions that have invested much work into creating VRML content or user interfaces.
The open-source model is well suited for minor contributions or one-semester projects with limited scope and capacity, rewarding the students with a complete and usable scaffold for their undertakings. The source code is there to read and learn from - FreeWRL in particular offers some very nicely structured standard 3D graphics code, such as the exceptionally well-documented quaternion routines (FreeWRL-1.01/CFuncs/quaternion.c).
All in all, open-source VR software on Linux is becoming a viable alternative to commercial closed-source (and possibly short-lived) offerings. In user interface research, having the source to drivers is critical. For VR research, FreeWRL's External Authoring Interface (EAI) and Java/Javascript support present interesting possibilities. On the user side, there is reasonable hope that the FreeWRL browser plugin will stabilize enough to seep into the major distributions, so that VRML97 will eventually become standard on every Linux desktop.