I will give only Solaris 10 and Solaris 8 examples, though I know it happens on 9 as well. It doesn't seem to matter what hardware or what patch level of the OS - it just doesn't seem to want to run stably.

Solaris 10:
SunOS (REDACTED) 5.10 Generic_138888-08 sun4v sparc SUNW,SPARC-Enterprise-T5120
grep "agentd started" zabbix_agentd.log
16568:20091228:230131.312 zabbix_agentd started. Zabbix 1.8 (revision 8564).
26399:20100119:171115.954 zabbix_agentd started. Zabbix 1.8 (revision 8564).
10599:20100130:205728.778 zabbix_agentd started. Zabbix 1.8 (revision 8564).
22135:20100202:173315.219 zabbix_agentd started. Zabbix 1.8 (revision 8564).
22190:20100202:173701.069 zabbix_agentd started. Zabbix 1.8 (revision 8564).
22414:20100202:173800.645 zabbix_agentd started. Zabbix 1.8 (revision 8564).

Solaris 8:
SunOS (REDACTED) 5.8 Generic_117350-51 sun4u sparc SUNW,UltraAX-i2
9276:20100201:001400.772 zabbix_agentd started. Zabbix 1.8 (revision 8564).
17237:20100201:023100.582 zabbix_agentd started. Zabbix 1.8 (revision 8564).
6580:20100201:080401.579 zabbix_agentd started. Zabbix 1.8 (revision 8564).
10654:20100201:091501.058 zabbix_agentd started. Zabbix 1.8 (revision 8564).
29844:20100201:144800.874 zabbix_agentd started. Zabbix 1.8 (revision 8564).
12832:20100202:022201.491 zabbix_agentd started. Zabbix 1.8 (revision 8564).
22456:20100202:134301.267 zabbix_agentd started. Zabbix 1.8 (revision 8564).
24875:20100202:142600.878 zabbix_agentd started. Zabbix 1.8 (revision 8564).
28448:20100202:152701.361 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).
1737:20100202:162101.616 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).
15648:20100202:202200.902 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).
8492:20100203:025500.664 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).
10496:20100203:032801.182 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).
24786:20100203:073500.975 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).
9769:20100203:202901.213 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).
12759:20100203:212100.637 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).
16076:20100204:065300.761 zabbix_agentd started. Zabbix 1.8.1 (revision 9702).

I had high hopes that the Zabbix 1.8.1 binaries would correct this but I was disappointed when it did not. As you can see, it can happen with stunning frequency (We have other Solaris 8 boxes that do this less frequently). I have tried this with binaries I've compiled myself and with the provided ones. Both have this problem.

I have a work around now: A cron job that runs every minute and tries to re-launch it if it failed. This is not an ideal solution, but it's worked for us so far. We prefer a more stable binary.