Subject: not all versions of Linux provide the right versions of libs, java and so on…
Domino can only use what it gets from the OS. For example if you run Domino on WinNT your get less performance than on Win2000 or Win2003. Each different version of a Operating System has own features.
In case of Linux it is mainly the kernel, the thread libs, java and some other resources in this area.
RH and SuSE are the tested versions. As soon you have a enterprise ready software you have to focus on some releases and distributions to get it supported and to use the features that give you performance but which might not be available in other versions.
So if you are using LD_ASSUME_KERNEL you are using old thread/memory model which does not give you and good scalability. For RH there are some problems with multiple CPUs in some versions (check the table for compatibility in the other post). For SuSE Enterprise Server 8 (first UnitedLinux) there is a special expansion pack that also works on SuSE 8.1 professional (as far I tested it and looked into the libs).
It is basically some new libs which are compiled in a different way and a set of changes to support to get more resources from the OS. Most of the settings are in our session we did last Lotusphere (check http://ldd.nashcom.de/nshweb/pages/lotusphere.htm) and you can download the UnitedLinux Domino Performance Pack from SuSE for free…
http://www.suse.com/sles/domino.html
The package has been jointly developed by IBM and SuSE.
SuSE does also provide Linux Versions for pSeries (AIX), iSeries (AS/400) and zSeries (S/390).
There is a version of Domino 6.5 which runs on zLinux and has a some specific kernel changes to support IOCP.
Those changes should make it into the Linux on Intel versions of Domino in the next feature release and will give Domino on Linux a performance and scalability boost far beyond anything you have ever seen on Win32 
But this needs the right support in the OS level. IOCP API support is there for almost any other platform than Linux. It has been back-ported to zLinux with the 2.4 kernel for Domino 6.5 and it will be available in the 2.6 kernel.
My personal recommendation is to use SuSE Linux. They have the best OS-level support for Domino and there are specific offerings for Domino like the link I posted in this document.
Sadly I have not seen much commitment lately from RedHat. I am still using RH 7.2 on my C-API build machines for Linux because this is the only supported platform for the C-API for D6.x. But everything else is SuSE here and that is also my recommendation for customer environments.
I hope this helps…
Daniel (ldd@nashcom.de)