This is an iMac G5, first-generation desktop … the first flat-screen model … running Panther (10.3.9) with any 3-button mouse. There is of course no “trackpad.” It’s got an nVidia card with 64MB VRAM.
Middle-mouse wheel works fine, but the moment you click it, you can’t stop dragging the view around.
If Emulate 3-button mouse is off, you are hosed: click the MMB and you can’t get out. However, if it is on, the effect can be arrested by Shift+LMB, that is, by emulating another MMB. This is, of course, a piss-poor workaround…
This has been discussed before…
http://www.blender.org/forum/viewtopic.php?t=4959 (here, something about “admin accounts” was mentioned, although I can’t imagine how that could actually be relevant)
I’ll be upgrading the OS on this machine soon, and will report if the problem goes-away at that time.