OK, so I tried this, but ECHO %PATH% just outputs %PATH%. I'm running DOS 6.22 on a 286. Same thing happens for any other variable defined in AUTOEXEC...
Confirmed. It doesn't work directly from the prompt, only within a batch file. I think might work on a DOS 7 prompt (Win95 >) though, not sure, I don't have a Windows machine hooked up atm. Maybe that's where my memory is from...