With Java being 100% platform independent its a solid investment to learn and the base code files are free ..just a time investment for the learning curve as opposed to monetary one <althou I know that time can be worth a lot more than money>..and yes you have more geeks and artsys type in traveller than in the general population. #2 with java being platform independent you dont have to rewrite the code year after year as the platform market changes or as windows makes an upgrade that makes the previous stuff done in v basic or .net obsolete ...
While 100% on all platforms, the libraries are not always there or the same across all platforms, and in my very limited experience it is write once, debug everywhere - especially the UI. But that was my very limited experience. Just like MS - sometimes the libraries get updated on some platforms and not others. Software, since the advent of the PC, is very fluidic. And it can't keep up with the hardware at all.
Bottom line it depends on your expected market & what you already have: there are probably a lot more Mac/Linux people than the general public in this forum since RPGs, particularly Traveller, seem to attract the more 'artistic' & 'geeky' types as noted, so this market may be more skewed towards that end. As previously noted, the largest market share is Windows, and while that share is going downhill, for the next 5-10 years, in businesses at least for personal production software, will remain the dominent market force and the most popular consumer-level OS. But if you currently just have a PC, then you may have a problem testing it on a Mac, and supporting software on multiple OS is a nightmare (heck, maintaining multiple versions of software for the same platform is equally horrible!)
But then - learn java & you could write it for MS' OS as well as the Apple crowd, and depending on the UI interface, for phones/PDAs running java. I'm just not convinced that the language is write once/deploy everywhere.
As an aside: even web-based things don't work the same across browsers: the major browsers each use different rendering engines and 'interpret' the HTML standards different, sometimes greviously so. The in-house apps I've done just look different on FF vs IE6 vs IE7 vs Slimbrowser (which is supposed to be IE6 based if I recall). I don't program for a specific browser since that market is also fractured, I just try to program to standards.
As a 2nd aside: the .net stuff is backwards compatible as well since any installer will also verify you've got the correct version running. There is also a partial Linux port of it. And heck - virtual PCs are going to be the way to go anyway: you can run Mac software on a PC or PC software on a Mac with a virtual PC. Bad news is that you still have to buy both OS...so for most that is not a do-able thing.
As a 3rd aside - no, just kidding. That was enough, and hopefully I've not derailed this conversation overly much.