Here is a sample Marvin code that converts an infix expression into postfix. I have also posted this one to the InfixComparison page of Concatenative Languages Wiki. Note from the future: Sadly, these links are broken.
# infix.mar | Converts an infix expression into postfix
# This implementation uses the global variable space as a hash-table
# for operator precedence.
"listop forth dequeop" use
@|> globs & pushfrom ;
@string dfront NONE =
{ iftrue dpushf }
{ else
dpopf 2dup |>
{ iftrue swap dpushf }