They probably used a RIP. with 6MP your pushing the envelope pretty hard. I wouldnt go past 20" x 30" with a shot containing a lot of detail....but we are testing a new RIP on the 60" wide printer next week. That might change everything.
another great shot :) I've had about 12 of my D70 pictures printed 30x45 but it was done by a local company in Philly so don't know the software used to upsize or print it.