We've recently done a project where we needed to do 3D visualizations in the browser. It's not incredibly fancy, mostly showing walls and floors of rooms and some stuff in them.
Before we started coding, we were discussing what platform to run it on. After some going back and forth, we ended up deciding that despite the 3D requirement, going for a web app would probably pay off in the long run because we then don't have the hassle of installed software and perhaps worse, the problem of predicting what particular platform Microsoft will declare obsolete next year.
So in the end, we settled on the web, going for HTML and canvas for the 2D part and WebGL, the new OpenGL standard for the web, for the 3D.
There's only one problem, WebGL was so new that at the time, only Chrome had a release with support. Not long after, Firefox joined the ranks. But still, it's a technology you can't rely on unless you can control the user base and ensure they install the right browser.
Similar problems kept the 2D HTML canvas back until a project like excanvas turned up. Until Internet Explorer support is there, new standards are a no-go for many.
Thus Martin has been working on and has now released JebGL, a WebGL emulation layer for older browsers and most importantly Internet Explorer, back to IE 6.
It relies on the OpenGL support in Java, translating WebGL calls into JOGL calls in a Java applet. We've had a look at other options, such as using the Flash 3D API, but we needed support for some of the fancy stuff in OpenGL so it would be a pretty huge task to try to emulate that in a completely different API (on a lower level, that's what Google is doing with the ANGLE project to try to work-around bad OpenGL drivers).
So as long as Java is in, you can now rely on WebGL (well, barring bugs and missing features in JebGL). And if Java is not there, there's at least the option of asking people to install it, most browsers seem to handle that gracefully these days, rather than requiring them to switch to a completely different browser.