try removing your doctype.
the easiest way i've found to do a fully centered block or image is to use the table within a table trick.
set the outer table's width AND height to 100%. setup one table row and one table cell. set the valign for that cell to middle and align to center.
inside that table cell, setup the inner table for your content.
keep in mind, this is a hackjob way of doing it so if you are concerned about validating your html, it won't fully validate. (height definition for table is not CSS compliant.... or something like that). you also need to remove your doctype or set it to loose or transitional.
here's an example:
http://www.disivion.com/songs/listing.html