Hi Adam,
That's a really good question! From my experience working as an in-house architecture visualiser. There is very little prospect for you career to evolve, imo. Unless the firm you work in is expanding and the visualisation department can grow, you can become the "head" of the visualisation department. If you work for a visualisation firm, it's pretty much as Justin says. Although, you would most likely specialise in some field, ie modelling, lighting, compositing.
The 3rd option is to setup on your own (guess which one I've chosen!) and yes it's tough, but the potential are enormous, since you can promote yourself straight to "overlord" , and it all just depends on what relationship you can establish with you clients.
I hope that helps, and good luck!
cheers
Fabrice
