It's good source when the measured timeframe is really long like since launch, but not so good when the timeframe is short and could include some lower than normal supply numbers due to channel clearance. Someone can correct me if I'm wrong, but I'm thinking this 12 month number zed was throwing out there is a pre 360 slim shipping numbers and naturally the volume was low just before that. Zed often likes to forget something like that.
edit: If that 3.9 million zed said was in relation to NA shipment numbers, which it probably is considering the context, then I'm quessing it includes one dry pre slim quarter for 360 AND one juicy slim PS3 quarter, the one where it launched. That skews the shipped numbers quite a bit... Definitely not an accurate picture. As far as I know 360 has outsold PS3 during these 12 months at retail in NA.
NPD lists the last year (sept 09 through august 10) as about 5.1 million 360 sales and about 4.4 million ps3 sales. Shipped numbers can be all over the grid depending on what they are doing regarding new sku's. They will even sometimes just flood the market to reach certain goals for a quarterly report or something. Without a lot of context the amount shipped in a period tells you very little.