Ok, if you're using IMAP Backend you will have no HTML support on your iOS Devices. To get html Support feel free to use the Z-Push AS12.1 branch that you can find in SVN hosted currently on BerliOS. Please use the search function of this board to find some details on how to get the branch from svn or read at berlios of svn checkout works.
AS12.1 Branch is not being supported as stated already by Manfred/Sebastian/Zarafa. If you want to use it please test it prior on a different v-host with your devices in case you're using Z-Push in a mission ciritcal commercial environment.
Using Zarafa 7.0.7b1 with my tuning patches, Z-Push AS12.1 Branch with GZip, SMS, SMime and HTML Support. Patches are available upon request and being announced on #IRC.