Here's a great tip if you're wanting a little more flexibility with your input devices under IRIX. As of IRIX 6.5.16, support for multi-button mice (including scroll wheel) has been added, though it's not active by default.
Here are the possible systune settings for pcmouse_mode:
0 - Default 3-button mode
1 - 3-button Wheel Mouse (type 3), Wheel generates buttons 4 & 5.
2 - 5-button Wheel Mouse (type 4), Wheel & buttons 4 & 5 generate button events 4 & 5.
Since I am using a Intellimouse Explorer, I chose '2'. The command to activate support in my case would then be (as root):
systune pcmouse_mode 2
After a reboot both the scroll wheel and middle button should be active. On a five button mouse such as the Intellimouse Explorer, the two additional buttons function as scroll up/down. You should also see an entry in /var/adm/SYSLOG like this:
Dec 15 20:19:07 5A:Mika unix: NOTICE: pcmouse: type=4
I've found the scroll wheel works wonderfully with Mozilla, SkipStone and many freeware applications available on http://freeware.sgi.com. If you use aterm as your default terminal it works with that as well.