I think this comes from Todd Howard stating at E3 he had "no idea" when the PS3/PC launch would follow after the 360. What that actually means, we'll see (probably nothing).
You need to consider that any bugs found in the beta must be fixed and retested, some things may need rebalancing, or there may be larger glitches that take longer to fix. So, it is possible that the beta may need to be delayed on the xbox, pushing the other releases back even further. Then there is always the chance that the DLC may not run properly on the PS3 or PC.