Mandriva is only providing their latest distribution .iso files to their club members (once again) but you can upgrade from Mandriva Linux 2005 to 2006 easily enough via urpmi (the Mandrake, er... Mandriva automated package manager) and ftp--if you have a fast connection. The following demonstrates upgrading for the x86_64 (AMD64) platform. You should be able to substitute /i586/ for /x86_64/ below if your are running a 32-bit processor but I didn't test it...
My apologies if the formatting for the commands listed below looks terrible in your browser. The URLs are too long for this template, so I have set them to the smallest font size. (I need to look into alternative blogware or another template.) You should be able to copy & paste the commands easily enough however.
[Updated October 16, 2005. Using a new template, URLs should display much better now?]
Open a shell and use the following commands (all one line for each command):
urpmi.addmedia --wget ftp_main ftp://mirrors.secsup.org/pub/linux/mandrakelinux/official/2006.0/x86_64/media/main/ with ../media_info/hdlist_main.cz
urpmi.addmedia --wget ftp_contrib ftp://mirrors.secsup.org/pub/linux/mandrakelinux/official/2006.0/x86_64/media/contrib/ with ../media_info/hdlist_contrib.cz
urpmi.addmedia --wget ftp_compat32 ftp://mirrors.secsup.org/pub/linux/mandrakelinux/official/2006.0/x86_64/media/compat_main32/ with ../media_info/hdlist_compat_main32.cz
urpmi.addmedia --wget --update ftp_update ftp://mirrors.secsup.org/pub/linux/mandrakelinux/official/updates/x86_64/2006.0/main_updates/ with ./media_info/hdlist.cz
urpmi --auto --auto-select
If you don't have wget installed you can leave off the --wget option in the commands listed.
That's it, reboot and enjoy your new Mandriva Linux distribution.
Disclaimer: Use the above information at your own risk. If you ruin your Mandriva install or otherwise mess up your system, that is your business, not mine.
[Updated 31 October 2005: Corrected entry for ftp_updates.]