I don't know about it being a fashion "trend" or "fad", largely because I never pay attention to that kind of thing. If I like it, I wear it... if I don't, I don't.
I have a neon green wristband from the Chicago AIDS Foundation, a red one I have to wear for work (its a company thing), and a black/white/red/yellow one that says "We are all related" that I got to support a Native American charity... and I wear all three. If I had my way about it, I'd ditch the red one because three wristbands are just too many and I actually like the other two...
There are other charities that I support that also have the wristbands, though I've never purchased any of them. I already have three, I don't need more. I think its a good way to raise funds and awareness, because they're usually not expensive for the charity to produce and they sell them fairly cheaply... and people tend to ask about them when they see them.
On the other hand, I'd much rather donate money when I have it, or donate time. I'd also much rather attend fund-raising functions (such as the brunch/bowling function I attend for a Lupus charity). And while its difficult to find information on exactly how the money is used, it can be found in many cases if you dig far enough. I tend to only support charities that make that information readily available.