Yes, you can!
To do this:
1. Log in to SMS Manager - http://sms.neural.net.au/
2. Select the SMS account to use and click "Switch Service" (if applicable - only if you have more than 1 SMS service)
2. Click Outbound (at the top)
3. Enter the date for the message to be sent in the "Schedule message" box in the format of YEAR-MONTH-DAY (eg 2012-01-01)
4. Enter the time in 24-hour format in the boxes to the right (eg. 13 : 00 for 1:00PM)
5. Fill out the other fields as required and click Send
Your message will now appear in the outbound list with the status of "Scheduled" and will be dispatched at the time requested.