With the 1.8.5 Zabbix agent, some of our hosts show 0% idle CPU on Zabbix while vmstat and top report >0% idle.
Downgrading to the 1.8.4 agent makes the problem go away.

This only seems to be a problem on hosts with non-contiguous CPU IDs, e.g.:

OK:
—
$ psrinfo
0 on-line since 12/01/2009 19:19:52
1 on-line since 12/01/2009 19:19:50
—

Not OK:
—
$ psrinfo
0 on-line since 03/19/2011 15:50:14
1 on-line since 03/19/2011 15:50:14
2 on-line since 03/19/2011 15:50:14
3 on-line since 03/19/2011 15:50:11
16 on-line since 03/19/2011 15:50:14
17 on-line since 03/19/2011 15:50:14
18 on-line since 03/19/2011 15:50:14
19 on-line since 03/19/2011 15:50:14
—