ATG Commerce (now Oracle ATG Commerce) is the niche I was speaking about, but the above list is 100% true as well. Lots and lots of opportunities.
I'm employed by an eCommerce vendor myself, so I'm sure I'd have an intuitive understanding of general architecture, but actually understanding the internals of a system as an outsider seems like a difficult proposition.
The only real hope of learning it though is to have a company that you work for use ATG, send you to training, and get you coding on it every day.
This is why there is such a huge gap between the demand and supply of this skill set currently.
But of course, rumours are unconfirmed.