MP3 is a lossy compression format, and as such, it by definition has to
throw away part of the audio signal, never to be recovered.  It uses a
frequency transform as part of the compression, which leads to certain
side effects.  So the upshot is the following, in my opinion, compared
to uncompressed 44.1:

-General loss of high frequencies.  These are considered less crucial.
-Less accuracy in mid and high frequencies.  These are places where
approximations for compression take their toll.
-A small amount of time smearing for transients in general.  This is due
to the effect of removing sound components from a frequency transform.
-Some loss of dynamic range.

	These effects become more severe as the amount of compression
increases.  Anyone who tells you that MP3 is "CD Quality" is fooling
themselves.  I would compare it more closely to FM broadcast, minus the
severe loss of dynamic range associated with FM compression.

	There also remain legal issues surrounding MP3.  The process is
proprietary and owner by Fraunhofer (sp?).  They have not been enforcing
their legal rights surrounding the players in an attempt to force a de
facto standard.  However, just recently they have been making
indications that they are going to start going after the free
encoder-writers and charge them a license fee.  The decoders will likely
remain free.  
-- 
Mike Berry
mikeb@nmol.com
http://www.nmol.com/users/mikeb