The great site iFixit has done the tear down of the new MacBook Pro with a Retina Display that was announced at the 2012 WWDC, and we find that it has more in common with the MacBook Air and iPad than any MacBook Pro before it; in other words, the RAM is SOLDERED in place. This means that you had better order as much as you will ever need now, as you will be stuck with what you have installed. Furthermore, the SSD is yet another proprietary format. So upgrading it in the future is going to require someone to make an upgrade which Apple will likely not bless and possibly sue out of existence.
I would understand if this was a MacBook Air. However this is supposed to be a product marketed to people who attend WWDC primarily. Developers commonly need to upgrade RAM and storage at some point. My advice: If you only think you need 8 GB, get 16 GB instead, and go for as much SSD storage as you can afford.
All of this doesn’t mean that it is not an amazing machine or not worth buying. It is just that buyers should know that this limitation exists. As for myself, the expandability is something I need, but if I really wanted the Retina Display, I would still buy it.