Where ctrl-M while in math mode used to insert \textrm{...}, it now inserts \mbox{...} (and nests mboxes if you keep using it). If you use ctrl-M to go into text mode and right-arrow to escape back to math mode, I think you will get what you ultimately intend, but with a shallower nesting depth (unless you literally need to have text nested in an equation nested in text ...). For instance, your example above will be rendered into LaTeX as $y=0,\mbox{ if }x<0$./Paul
That's the workaround I use for now, but I'm actually looking for the nested solution. Is there any way to reproduce the old behavior (short of exporting to LaTeX and manually tweaking)? Asides: are there any benefits to \mbox over \textrm? It would be cool if we could someday use \text, \intertext, etc. from the amstext package.
