I am wanting to set up a production application server using JBoss (written in Java) and PostgreSQL (written in C I believe), so would be interested in opinions as to what the most reliable platform would be for this. Firstly, I am wondering what flavour of Linux is considered the most reliable and what version. I am currently using Red Hat 8.0. Secondly, I am would be interested if people have experience with different JVMs for such a application.

When it gets down to it, linux is linux. That said, different distros offer certain charactertistics which you may prefer. Debian Woody (3.0), for example, offers a very stable set of packages. This is good in that you are running tried and tested packages throughout - but on the flip side, if you really need something that is in very recent release of a package (ie, something in the latest gcc, or postgres, or so on), then it becomes more of a headache to use this with Debian. You could use Debian's unstable target, however I really wouldn't recommend this for a production server. RedHat, as other people have mentioned, seems to be a fairly good candidate. If you're really worried about it being a production server, it seems that going for their enterprise-class distro is a better bet, and offers better support. If you really want bleeding edge, you can go for slackware or gentoo. You can always go for FreeBSD as well, which seems to be a fairly popular candidate for a stable production server, although if you're used to running a linux system it might be a fair jump to start using FreeBSD

Perhaps another way of asking the same question: Are there any platforms or combination of platforms that should be avoided.

That is a better way of asking, but as I pointed out about it depends on what you want out of your distro. Personally, I'd use Debian Woody, as thats what I'm used to. However, if you're used to RedHat, I'd suggest you stick with it, perhaps following Olivers recommendations. Daniel