Hey mate.. I know you have moved on fronm this issue but for everyone's information, the timezone codes lord_hyrax got is from the old article. At some stage between XP and Vista (also server 2k3 to 2k8), the timezone codes were renamed so you need to specify the OS you want to target. You also need to be be very careful when reading articles about timezones because they change very often due to some states implementing/extending DST and so on. Not sure if you can refer to the timezone via the code when using the OSDTimeZone variable but you can defintely use TZ Codes using sysprep.
http://technet.microsoft.com/en-us/library/cc749073(v=ws.10).aspx
Regards,
tupham