Called today to activate my recent 3x500 unactivated orders. They were shipped on the 26th and 28th respectively. Didn't try to use or check balance until the 4th. GCM phone rep said they can take up to 4 days to activate once mailed and they were "blocked" by my attempting to use them before they were activated (in both cases clearly well over 4 days since mailing). She said to call back their customer service in a few hours to see if they had been activated - if I try to check balance online it might block them again! Blegh I wish they would get this sorted out.
Also, FWIW, I used the rest of my GCM FBs yesterday without needing to call to activate (despite what is indicated on the letter they arrived with).