Looks like there's a driver for 10.4 (Tiger) from 2005 so it's a bit old. Nothing for Leopard yet. If your iMac is on Tiger you're set:
http://h10025.www1.hp.com/ewfrf/wc/s...os=219&lang=en
If not, here's some hp printers that are Leopard compatible:
www4.hp.com/mac_connect/pdf/Mac_OS_launch_special.pdf
According to that pdf HP is putting out software for All in One printers (like yours) in December 2007 to update printers that aren't automatically included in Leopard. If your printer doesn't work now, it might work by the end of the month if you update your computer regularly.