The rumor is that the 60gb model will replace the 20gb model as the standard "premium" model..

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And put the remaining 20gb HDDs left in the Arcades. But thats even moreso a rumor. The 60gb rumor almost a definate, since 20gb HDDs are not very cheap to make vs. a 60gb at the moment.

its confirmed
http://n4g.com/News-120115.aspx

http://n4g.com/xbox360/News-120157.aspx

I believe in the new premium 60gb model

but I doubt there will be a new arcade with a 20gb hhd

....Could MS be able to launch a 60gb Premium model + price drop before GTAIV?

It would be tough but if that's their plan, I hope it works for them. They need a real shot in the arm for GTAIV but after one lousy $50 price cut in 2007, let's just say that I'm not the most optimistic person in the world when it comes to the Xbox division and its competence.

Your first link is about blu-ray, and the second is just n4g reporting the same rumour as the OP.

It would make sense though for the 20gig Premium to replace the Arcade SKU. It's just too bad that there are 360s out there without a hard drive.

I wouldn't worry too much about lack of an HD on all models if I were a software publisher.  If a 360 Arcade owner really wants a particular game that requires a hard drive, it can be added.  It's not like the low-end PS3s that were missing wireless but it couldn't be added, missing BC hardware that couldn't be added, etc. 

I am thinking about a 360, although i would not say this is a massive factor to draw me in