I don't think most people are putting their thinking for this issue in the right perspective.
In regards to comparing it to other launches, you can't. The PS3 had more supply than demand, that is the biggest issue here. Therefore there weren't people waiting for shipments to come in. If you look at historical data the other consoles took as much of a plummet (maybe not quite as much) in their second week, but the demand was still huge... X360 and Wii were both considered 'sold out' for weeks after they were released... the Wii is STILL sold out.
I seriously wonder the number the Wii could have been pulling if there was more supply, I think Nintendo may have killed some demand because of leaving supply levels too low for too long.
To put this in perspective, I think the console sold exceedingly well for it's first week (as someone said it's like the launch of a videogame up huge at the start, and then plummets down below the weeks after) and then it will continue to drop slightly week on week after this. This isn't good for sony because it follows the typical hardcore gamer trend, and therefore without any new reasons to purchase games the weekly sales would continue to drop. I would say 15k a week would be a good estimate as to what it will reach.
For once I agree with Hus about 30k a week being bad sales, it is bad ESPECIALLY in it's second week, no way will week 3 be that high, demand isn't as high as people think that it is - ongoing.
Honestly, I think that the system needs to KEEP atleast 30 - 40k a week to not follow the rest of the worlds current sales trend for that console (which I expect it will anyway - what reason is there in the UK that it will not?)
Essay Pro