The also redid their scaffolding model (grid system) to use percent widths instead of pixel widths, so it seems like the default (and only option) will be for things to behave like the old fluid layouts did.
Yup, and then if you need specific pixel widths on anything, you specify them yourself. E.g. a fixed width page, you'd need `body { width:950px }` or etc.
However I am not looking forward to removing all the row-fluid/container-fluids in my templates.
You should be able to use find-and-replace to tackle most of this—the classes aren't changing too much.