STLport is not used much nowadays. Used to use it with msvc-6 and 7.1, but those are loooong gone here.
Keep in mind that msvc is not the only place someone might consider using a more portable STL implementation. Where I work we're using Sun Studio; at least in old implementations their support for STL was anything but standards compliant -- in particular, people were doing god-awful things that never should have passed peer review; stuff that stlport or gcc puke on. Using stlport in older versions of Sun Studio is all but necessary if you want to use boost. More recent releases of Sun Studio are less troublesome, but in our case we're porting between Solaris and Linux; the implementations of STL between SS for Solaris & SS for Linux appeared to have different behaviors for some things, and if we port to gcc there'll no doubt be issues with poorly written code that worked in SS that doesn't work under gcc. In short, I wouldn't discount stlport as an ancient, no-longer-used piece of software; for those of us trying to port/improve older and more fragile code, stlport is a boon. -Brian