While this is certainly true, a lot of devices won't charge simply because the vendor has taken steps to try to only charge from "officially sanctioned" chargers. (Read: they want to force you to buy from them)

If you look around, some chargers are better than others at being accepted by various devices. Also, you can sometimes find cables or dongles that will allow various devices to be charged.

Examples:

While my various iPod devices will charge using a number of "line-out" style cables with my extended battery pack, my Verizon iPhone 4 will only charge if I use a SendStation PocketDock Mini dongle as a line-out enabled cable (even tho it complains about it):

http://www.amazon.com/gp/product/B002ACNZFI

I was never able to get my Motorola Razr V3m to charge from my battery pack regardless of the cable, but I was able to find a thingy that allowed me to charge the battery:

http://www.amazon.com/gp/product/B002MVQFMW

Here is the battery pack I'm using these days:

http://www.amazon.com/gp/product/B004OC0122

In general, I've found it a mix of research and trial and error and luck. Also, note that now that USB has more widespread appeal, I think newer devices are more likely to allow third party chargers.

Of course the rub is, as Martin pointed out, a lot of newer devices draw more power and are more likely to run into the USB 2.0 standard issues. On the other hand, more and more chargers are being built to charge devices wanting extra power.

The moral of the story is -- don't just throw a charger in your bag for emergencies and assume it will work for your devices! :-)

Also, if planning for emergencies, keep in mind that sometimes you may need to charge more than one device concurrently. Think through the logistics, keep in mind how long it takes to charge a device, how fast they are depleted, how many devices are likely to be on at once.

-john