The main website for the OpenPsion project is: OpenPsion. The Files section at Sourceforge, OpenPsion Files has Arlo, kernels, initrd's, the X-server available for downloading, together with a few applications.
The 5MX Pages of OpenPsion has several links with useful information on linux on the 5MX.
This is the easiest program to transfer files from you linux desktop to the psion, pretty much plug-n-play. PLP Tools at Source Forge
Zipped source and binary files can now be downloaded from the OpenPsion web page Arlo Files
The official ARLO page is located at Peter van Sebille's webpage
The ARLO HOWTO is HERE
The proboot utility allows the 5MX PRO to be set up to boot linux directly from the compactflash card. The 5MX PRO machines have a 128 byte EEPROM that loads the sys$rom.bin from the flash card; see the README. Proboot does NOT work with the standard Psion 5mx, or Revo, please use ArLo bootloader for those instead.
I believe that as of 5/06 OpenPsion has adapted kernel 2.4.27 as their standard kernel, with 2.4.32 being the development kernel. Kernel 2.4.19 works fairly well as well.
The 2.4.27 kernel, patched for the 5MX, can be downloaded from http://svn.exactcode.de/linux24-psionw/ using subversion.
svn co http://svn.exactcode.de/linux24-psionw/ linux24-psionwSubversion is a Debian package. I think this site tries to keep the 5MX kernel patchs uptodate.
The 2.4.32 development kernel can be downloaded from http://www.woodall.me.uk/psion/psion.html where you can also get kernel 2.4.19.
The source for the 5MX kernels for the longest time was www.muru.com by Tony L., but development there seems to have ceased.
Kernels for the 5MX have been made available by Tony Lindgren. Try The Files section for OpenPsion at Source Forge. Note that the kernels for the 5MX must be uncompressed.
I don't actually know of a place to get a good solid initrd for the 5MX at the moment; this seems to be a perenial problem. One of these initrd's can be found at The Distributions for the 5MX Directory at Source Forge (the file 5mx-linux-boot.tar.gz, which is not a very good one, but perhaps adequate.) The dpkg distribution tarballs "disk1.tar.gz", obtainable from the the Arlo section at Sourceforge Files, has an initrd in the package already.
You can post queries at the OpenPsion mail list by sending an e-mail to linux-7110-psion@lists.sourceforge.net. You might even get a reply either directly, or appearing on the mail list.
You can browse the mail list archives, including the e-mail you just sent, by going to https://sourceforge.net/mailarchive/forum.php?forum_id=7163.
Psion 5MX's apparently have a common failure with the flexible cable connecting the unit to the LCD. Typical symptoms are a screenful of horizontal lines. If this cable fails there is often nothing to do but get a new psion (try ebay.com?); replacement cables seem to be impossible to find. Some hints on how to repair your broken psion, and perhaps even some hints at preventive maintenance can be found at The PDA Graveyard [Broken link...]. WARNING: The photos there are not for the faint of heart; rated R for gore and violence.