...and it's not a bad one 
First off, being suspicious, I backed up my O/S partition via DD to a file on the other partition, and then copied it to an external PC - just in case "it all went wrong".
Then, I pinched Volker's instructions from http://forums.openmediavault.org/viewtopic.php?f=12&t=51 with a few mods/extra steps...
1) Edit the file '/etc/apt/sources.list.d/openmediavault.list' and change the repository name from 'ix' to 'omnius'.
2) apt-get update
3) apt-get upgrade
Let it finish
Reboot
Login to WebUI - check most things work. Notice the 0.2 plugins (website/OpenVPN) are giving JSON errors.
4) Remove the 0.2 plugins updater forcibly, via apt-get remove openmediavault-omvpluginsorg
5) Grab the 0.3 plugins .deb file from http://omv-plugins.org/ - i.e. wget http://packages.omv-plugins.or…0.3.2~3.gbpa0b737_all.deb
6) dpkg -install openmediavault-omvpluginsorg_0.3.2~3.gbpa0b737_all.deb
7) apt-get update
apt-get upgrade
9) reboot
All well so far!
I will add, this is on a custom install - my O/S partition AND data partitions are both RAID1, and I have FreeRADIUS, Squid, Asterisk, DNSCrypt and other things all running too, so the box is quite central to my ops