Any time you deal with RTOS, on 8051 or other targets, the stack is the thing to take care of.

I will speak about our RTOS because I know it and I don't know the others very well, but I think that the others are quite similar.

Basically, the OS is scalable:
If you have a small application with few tasks with small stacks, then you use the 'tiny' version, which is limited but works even without XDATA.
When your application is bigger, there are 2 other versions of the OS (standard and large) which use the XDATA for dumping the stack _if_needed_.

So in the end it's just like if you were not using a RTOS: If you have a small app, you keep in the idata, and if you have a bigger app, you start using the xdata when needed. Once again, the only thing that the RTOS will change is the time needed for designing the app.

Vincent