I'm not sure how welcome Thread Necromancy is ("Thou Shalt Not Bump Beyond the Veil of Death"?) around here but I was actually thinking about this very topic the other day, and I thought that a list of what we do know about magic vs. technology from the various books might help those that were looking for one come up with a rule regarding that.

We do know that the more wizards there are, the less likely technology is to hold out. (SK)
We do know that technology shorts out in the presence of just about ANY supernatural occurance, even if wizard's aren't involved (FM, SK, I think it's mentioned in GP)
We do know that the emotional state of the wizard seems to have some effect on whether the technology will work (Several books, the best reference is DM).
We do know that if lots of magic is flying around, it's more likely that technology will fail, and that mages can focus their hex to some degree.
We do know that magic's effect on technology can be warded against and spells can be used to mitigate it. (DM, DB)

This indicates to me that the effect of magic on technology is a direct result of magic itself rather than the magician. This may seem circular, but what I'm essentially saying is that magic is, itself, a force. This seems to be borne out by some of Harry's musings on magic in the books. Being a force, magic may be similar in some ways to electromagnetic energy - not the same, certainly, but similar. This would explain why ghouls and things like that that aren't wizards still mess with cameras - they possess a natural "magical field", a sort of radiation generated by their magical natures and exacerbated by their emotional state (because magic is generated from emotion, after all). This holds true for wizards as well, and Harry himself has mentioned that wizards, especially emotional wizards, can generate a lot of excess energy - otherwise why would Mac's be built as it is?

Since the effect is generated by magic itself rather than some sort of electro-magical field or the subconscious whims of mages, we can then begin to extrapolate that manipulating the hex effect is as simple as affecting a change in the magic of an area. The key is to be very, very specific in how you affect the magic - so you don't get an impressive pyrotechnic display when you knock out a camera. This likely requires a great deal of subtlety - hence why Harry has a hard time with it. Another wizard with a little bit more finesse might actually be able to direct his field away from certain objects (much as Harry directs his field TOWARD certain objects - such as cameras), allowing him to carry weapons and gear of a more sensitive nature than would be expected for a wizard. I doubt that this is terribly common, not because it's difficult, but because wizards tend to think in terms of spells. The White Council, and its enemies, seem to have developed "Everything's a Nail" syndrome, so only the younger wizards take the time to develop the magical skills necessary to channel magic in such a way as to allow them to use grenades effectively.

In short (too late, right?), its the magic itself that's doing it. As to why? Well, magic is a naturally intuitive force - yes, it has rules, but those rules are always changing, and a magician can get by just fine on a bit of wisdom, a lot of instinct and a little luck. Technology, on the other hand, is based around science. Yes, there's room for inspiration, but it's mostly based on trial and error, logical progression and static understanding - which is to say that there's a finite amount of science and, once we've come to completely understand it all, there's no more to learn (not to say that understanding is even possible). Magic on the other hand is chaotic and ever-changing. Much as in the game Arcanum, magic could affect technology because the two are counter-intuitive - a sort of conflict between the left and right halves of the Universal Brain, so to speak.

Also, yay first post.