I personally would say progressive rock, is rock that pushes the envelope on traditional rock and roll conventions, while still appealing to a rock and roll conventionality. Standard modern drum synthesizer sounds don't really seem to fit the bill as progressive to me. Stuff I think is progressive.
Battles http://www.youtube.com/watch?v=3FsvMyQeC-Q
King Crimson http://www.youtube.com/watch?v=hCo00hwrlK0
Frank Zappa and the Mothers of Invention http://www.youtube.com/watch?v=K_95u4axtNY&feature=relat...
Captain Beefhart http://www.youtube.com/watch?v=1mi8vz_3tZc&feature=relmf...
Can http://www.youtube.com/watch?v=pQ7pYfELOeY Plenty of other, but I think that these are fairly representative of what I'm talking about.
I'll re-post some of these later, some good stuff that some people have probably never heard.
If I post them at this hour they will just disappear to page 2 or 3 of hubski, and never be seen by anyone.