Tested under Linux, agent compiled from trunk branch:

1. ./zabbix_agentd-r30117 -V
   Zabbix Agent (daemon) v2.1.0 (revision 30113) (21 May 2012)
   Compilation time: Oct 5 2012 15:14:43

the keys work

1. ./zabbix_agentd-r30118 -V
   Zabbix Agent (daemon) v2.1.0 (revision 30118) (21 May 2012)
   Compilation time: Oct 5 2012 15:16:47

the keys do not work

Debuglog:
17991:20121005:152304.961 Processing request.
17991:20121005:152304.961 Requested [vfs.dev.write[sda,ops]]
17991:20121005:152304.961 Sending back [ZBX_NOTSUPPORTED]

Broken in rev 30118 (for trunk), 30117 (2.0 branch) in ZBX-5473

Reported in a forum thread (in Russian): http://www.zabbix.com/forum/showthread.php?p=114707