So tomorrow, on the climax of our campaign, I plan to have the group's slicer attempt to hack into a prison's security network. This prompted an inquiry: is there a way to do slicing combat between the PC and the prison's security computer? How could this be made interesting? What would threats or triumphs mean?

See Special Modifications for exactly this... more or less ;-)

Slicing combat is sort of like space combat, in that it follows the same basic format as normal combat but has different actions you take and has different results. Like Azanael said, go to Special Modifications for the rules.

Assuming you don't want to go out and buy a new book, some brief ideas: Threats might indicate the slicer is leaving traces of themselves behind in the system that could serve as clues to their identity. Despair might result in tripping an automated alert that notifies security of the terminal they are slicing from so a security team can be dispatched. Triumph might allow the slicer to obscure any signs that there was an intrusion in the first place, or hit upon some negligent user's plain text file with all the door keycodes and passwords.

As for the meat and bones of the encounter, what is the group's slicer going to be getting into the system for? Are they going to be required to shut down security or open doors, or are they the ones who are going to tell the group which detention block they need to go to? Figure out what the slicer's objectives are going to be and break it down into meaningful chunks that you can assign skill checks to, such as Accessing the System (Hard Computers), then Disabling Network Security (Hard Computers upgraded with a destiny point and a setback), etc.