The wording, "space available" is basically equivalent to "standby"; i.e. if there's space available at departure time, you can be upgraded.

From the NW Elite website:

...Unlimited Confirmed First Class Upgrades on domestic Northwest- and Continental-operated flights, with no certificates required - and the ability to upgrade from any fare...

Note that "Midway Airlines" is not mentioned here at all for "confirmed" upgrades.