by definition, strictly speaking, an acronym IS a pronounceable word formed from (usually) the initial letters of words. when it is pronounced as the letters, it's called an initialism. (using non-initial letters really results in portmanteau words.)

Wikipedia has a fair gloss: http://en.wikipedia.org/wiki/Acronym

some more examples:
scuba
aids
and from the military, SNAFU, FUBAR, etc.

[I'm sure this must be YART material :) ]