Bug submission is an important aspect of many OpenSource projects, and submitting bugs correctly increases the chances of the core development team finding, and fixing any problems that may arise. Of important note is the realization that it may not be the GIMP causing your particular problem, thus, one can understand the need for clear, concise, and acurate submissions to the GIMP developers.
As outlined, it is prefered if you can supply a certain level of system information in you bug submissions. You may well be wondering what, or indeed how to get this all important information.
The first piece of information that is important is the version of the GIMP you are running. This can be found by clicking "Help", and then "About". As you can imagine, if the problem you are submitting has already been fixed in a more current version of the GIMP than what you are using - the solution will be very straight forward. It is also helpful to know where your copy of the GIMP orginated from. Was it included with your distribution? Did you copy it off a magazine CD? Did you download the .rpm's or the .deb's? This will help locate problems caused by external influences, like incorrect library versions. Of equal importance is the version of GTK currently in use on your system. The easiest way to find this information is to start a terminal window (Xterm, Eterm, konsole, etc), and run gtk-config --version. This will output your GTK version to the terminal.
Another number that may help, is your XFree86 version. This can be a little more difficult to find out. If you use a Linux distribution that uses a package management system such as RedHat, Mandrake, Debian, CorelLinux, or ome of the many others, you will find the package tools invaluable in discovering application version numbers. XFree86 is the windowing system that allows Window Managers to display such things as windows, titlebars, docks, panels, and all the other prettiness you may see on your screen. If you are using Windows, this does not apply.
If you have a problem related to windowing (such as dialogs not showing, or the GIMP suddenly disapearing without apparent error) or display oriented strangeness, it would be helpful to know which windows manager you are using. Another question that could be asked are, is whether GNOME or KDE (if either) are being used.
Another useful titbit of information that is valuable for diagnosis, is stating excactly what it is that has caused the problem. Simply saying, "The foo plugin doesn't work." is not very helpful. Try to be as explicit as possible, "When I clicked 'foo-button' in the 'bar-plugin' the GIMP told me that such_and_such didn't work.". If the developers don't know excactly which part of the GIMP caused the error, or problem you are refering to, they can't really help you.
Now that you have gathered all of the information needed for an accurate bug submission, let's move on to actually submitting the report.
There are two good ways to submit your report. The first is Xachs nice site. The other way to go is to send a post to the gimp-user mailinglist. Subscription information can be found here. The user list if a great place for GIMP users to talk about problems, and solutions they encounter.
Now that you've submitted a bug report that's concise, insightful, and just a little daring, what now? It would be unreasonable to expect the developers to respond to every report in person, but know that your report has been noted, and if it can be reproduced, will be fixed. Bugs that are not actual bugs (problems that are not related to code) should be answered for you if someone else can reproduce it, or has had the same problem before.