Fixed in dev branch: svn://svn.zabbix.com/branches/dev/ZBXNEXT-792

Available in pre-1.8.6 r19987 and pre-1.9.5 r19990.

Short description: added support of parameters for the "system.hostname" key.
Parameters: netbios (default) or host

Tested two Windows hosts. In bot cases used mixed letters (lowercase and uppercase) with long host name.
Note: NetBIOS host name is trimmed to 15 symbols on hostname saving.
In both cases, a cmd command "hostname" returns real (case sensitive), full host name.

Results:

Windows XP SP3 32bit:

> hostname
WinXP-234TesTnamELonG

> zabbix_get -s localhost -k system.hostname[host]
WinXP-234TesTnamELonG

> zabbix_get -s localhost -k system.hostname
WINXP-234TESTNA

> zabbix_get -s localhost -k system.uname
Windows WINXP-234TESTNA 5.1.2600 Windows XP Service Pack 3 Intel IA-32

> zabbix_get -s localhost -k agent.version
1.8.12rc1

Server 2008 R2 Standard 64bit:

> hostname
Win-Serv2008-I65A4BignamE

>zabbix_get -s localhost -k system.hostname[host]
Win-Serv2008-I65A4BignamE

> zabbix_get -s localhost -k system.hostname
WIN-SERV2008-I6

> zabbix_get -s localhost -k system.uname
Windows WIN-SERV2008-I6 6.1.7600 Windows Server 2008 Intel IA-32

> zabbix_get -s localhost -k agent.version
1.8.12rc1

Conclusions:
the key "system.hostname[host]" always returns full, real (case sensitive), full host name.
the key "system.hostname" always returns trimmed to 15 symbols and always in UPPERCASE host name (NetBIOS name).

To finalize this development nicely, two documentation pages were improved (only for 2.0 doc):
http://www.zabbix.com/documentation/2.0/manual/config/items/itemtypes/zabbix_agent
http://www.zabbix.com/documentation/2.0/manual/appendix/install/windows_agent#configuration