Use Magic Device skill - Change its usage...

[Kor - Orc Scrollkeeper]

The use of a CHA modifier for UMD has never seemed to make a lot of sense... wouldn't INT or WIS be a more appropriate ability modifier?

How about just incorporating UMD into "Knowledge Arcana"?

[Disciple of Sakura]

Charisma always made sense to me. UMD is about tricking a magic device into thinking you actually are able to use it. Just like lying to the king and convincing him that you're from the Holy Empire of Xat relies upon Charisma, fooling a magical device into thinking you really are a 20th level wizard with a 120 INT should rely upon the same stat.

Also, it makes Charisma slightly more useful, which is always a good thing. Dump-statting Charisma is a very common phenomenon, but you get such mileage out of UMD that making it tied to a much more beneficial stat would make it almost a gimme that anyone would take it.

[Paul Watson]

Also, in Pathfinder at least, Charisma seems to be the 'grab magic by the scruff of the neck and force it to do what you want' stat (aka Spontaneous casting and SLA ability mod). In that vein, Charisma is the perfect stat for getting an item to work with sheer will and effort.