I have done this in the past with no issues on 2600, 2900, and 3500 series switches, and it is exactly as martyn and bobcheese describe... that being said, the most recent install I did with a 2920 switch had these options but it did not work, I don't know if it's a firmware thing or HP changed the way this works, but after messing with it for a few hours I gave up and just added a second network connection on the customers router for that subnet and (un)tagged traffic on two ports of the switch, using the router as the way between the networks.
But back to the OP's question, remember when you set this up, the switch has to have IP addresses in each VLAN, assigned a gateway (your router's IP most likely), IP Routing turned on, then all devices in either subnet use the HP switch's IP address for each specific subnet as their gateway and not the normal router anymore, the HP switch hands off traffic to the gateway and routes between the two networks. It is pretty much that simple.
