A prefix notation calculator that asks about everything step by step
Use a shebang at the beginning



This is something like #!/usr/bin/env python2, and helps interpreters know how to run it.

Remove some of the unnecessary comments

Comments are good. Excessive comments are just hard to read.

e.g.

elif cmd == "+":
    x = (lorr_input("Left side addend? ", x) +
         lorr_input("Right side addend? ", x))
    print x # Implicitly output result


Turn that into:

elif cmd == "+":
    x = (lorr_input("Left side addend? ", x) +
         lorr_input("Right side addend? ", x))
    print x


And:

# Get user input
cmd = raw_input("&gt;&gt; ")
# Process input
if cmd == "quit":
    ....


Into:

cmd = raw_input("&gt;&gt; ")
# Process user input
if cmd == "quit":
    ....


Empty lines for readability

Using the previous example:

cmd = raw_input("&gt;&gt; ")
# Process user input
if cmd == "quit":
    ....


Into:

cmd = raw_input("&gt;&gt; ")

# Process user input
if cmd == "quit":
    ....


if __name__ == "__main__" check

This just checks if you are running this directly, or from a module import/something else.

Maybe put the while loop into a function called start() and use:

if __name__ == "__main__":
    start()


Don't assign unnecessary variables

The helpmsg var is pretty much useless, just print the plain string.

The lit_input function is not used

Either use it, or remove it. Maybe make a variation of the docstring for the lorr_input?

Other notes:


Nice job with the ast safe eval, but a method that doesn't involve eval would also be nice. This is a lot of work, probably unnecessary.
I would set x to 0 at the beginning, but that's personal preference.
I'm not sure, but you appear to have limited to 87 chars per line (or under). That is a PEP guideline, good job.