Tips on avoiding "Could not activate cellular data network" with New iPad

Solution 1:

I have had exactly the same issues with my New Ipad 16Gb 4G. The only way I have found that really gets round it, is to manually select the Network provider, insteady of leaving it set to the 'Automatic' setting. Come on Optus sort this out !!

Solution 2:

According to the website of Optus, all should be working after a sync with iTunes (this is about an iPhone, but I'm sure it applicable to an iPad as well):

iPhone
iPhone specific APNs and technical settings are automatically provisioned when you first connect your iPhone (with an Optus SIM card inserted) to iTunes and activate your service. Performing a restore will also resend these settings to your iPhone.

UPDATE:
Try to reset your network settings by going to Settings > General > Reset > Reset Network Settings