Best as I can tell from this thread:
https://geekhack.org/index.php?topic=56386.0
This uses the square Panasonic style switch, not a Omron DF2 style:
http://my.element14.com/panasonic-electronic-components/evqp0e07k/switch-6-2x6-2mm-0-74n/dp/2079570#ProductSubstitutes
Omron makes something similar (and 4x the cost) called a B3M-6009 tactile switch, scroll down on the link above and you will see that they interchange, however, don't presume the Omron to be of any better quality or longevity.
Either way, this should give you the info you need to replace it, looks like an easy trackball to work on.
Yeah, that part matches up visually with what I've got in my trackball. Good eye. And yeah, this is probably the easiest input device to disassemble that I've run across. Probably helps that it actually was intended to be worked on by the user.
Show Image
(http://emoticoner.com/files/emoticons/onion-head/uhuhuh-onion-head-emoticon.gif?1292862523)
Post a picture from TOP down (of the pcb) and Tp4 can tell you if it fits.
Also, check with the multi meter if it's a Normally (closed) or normally (open) switch.
My guess is, it's using the brown point switches like in the WMO, in which case you can refer to my WMO ultimate thread, because that has you cutting one of the leads to the D2F switch as well..
Link below
Thanks for pointing out your mod. I ordered some D2F-01 switches as well as the Panasonic ones Leslieann linked to. Between the two, I figure I'll get it done. I'm thinking of trying to mod my scroll wheel to be less loose as well.