I have seen posts from folks that have gotten Java to work, so it is definitely doable. And there is no reason why it wouldn't be doable. But a JVM needs to be written for a particular system. It is (as the name implies) the virtual machine that translates a standard java request (for example, accessing a file or obtaining memory) into the system-specific request for that system. So the JVM needs to be created for ARM (which it has) and that may need to be further tweaked to run under Maemo.

I'm not familiar with incredibleCharts, but theoretically anything can be ported to work on the N900, especially if it was already written for the Gnome variant of linux, and moreso if it was written with the QT framework or in Python. The question is how useable it will be on the small screen, small keyboard, slower processor (compared to your desktop computer), and more limited internet connection. You will see some ported programs (like KeepassX) which are fully useable on the N900 but which are not tweaked to use the local interface features. Other ports, such as Conboy, are heavily or completely rewritten to suit the small screen.

Chances are, though, if the app has "pro" in its name, it is not free or open source. The folks who wrote it will have to port it themselves or release the source code so that someone else can do it for them (if they have the urge).

I predict it won't be too long before there is a decent Java implementation for the N900. I also predict an android emulator to go with it, but don't hold your breath for that (and we'll see what access to the Android app store is allowed). I have no idea whether that might bring java applet support to the browser, but I don't see why not.