i upgraded today to version 6.0.4, still wasn't able to receive the calendar via caldav.
Then i found the problem.
My Zimbra is running behind a apache reverse Proxy.
and IPhone connects to /principals/users/username ... and got an error 404 (doesn't exist)
no i added /principals to proxy to the Zimbra
and now it work's as several weeks before.
this was also needed to the productive zimbra 5 Server.
did Apple change anything.
1x Release 6.0.10_GA_2692.UBUNTU8_64 NETWORK edition.
1x Release 6.0.14_GA_2928.UBUNTU8_64 NETWORK edition
2x Release 7.1.3_GA_3346.UBUNTU10_64 NETWORK edition, Patch 7.1.3_P1
1x Release 8.0.2.GA.5569.UBUNTU12.64 NETWORK edition
2x Release 6.0.10_GA_2692.UBUNTU8_64 FOSS edition
2x Release 7.2.2_GA_2852.UBUNTU10_64 FOSS edition
1x Release 7.1.4_GA_2555.UBUNTU10_64 FOSS
1x Release 8.0.3.GA.5664.UBUNTU12.64 FOSS edition