Zabbix seem to have problems with spaces and empty strings on my setup. E.g., "rk Agent" is treated as "rk"":

rudolfs@rudolfs-pc:~/zabbix/branches/ZBXNEXT-780$ zabbix_sender -z 192.168.3.27 -p 10051 -s "rk Agent" -k sendertest -o "a"
info from server: "Processed 0 Failed 1 Total 1 Seconds spent 0.000155"
sent: 1; skipped: 0; total: 1

6084:20110517:133906.658 Trapper got [{
"request":"sender data",
"data":[

{ "host":"rk", "key":"sendertest", "value":"a"}

]}] len 98