Short answer, yes, a 3rd party charger that has not been specifically designed for your phone can damage the battery and/or shorten its life.
You may be interested in this section of the wiki article (if you haven't already read it, since the whole article is pretty interesting):
http://en.wikipedia.org/wiki/Battery..._phone_charger
which also refers to another wiki article about USB power and standardization.
Myself, I would use a 3rd party charger only if it is made by or recommended by the manufacturer of my phone.