From Pioneer's CDJ-400 manual:
"The CDJ-400 may not support playback with all USB memory devices or provide sufficient electrical power...Erratic performance may result when a USB hub is used."
As one who has experienced the frustration of getting the CDJ-400 to recognize my USB memory device; and who has, since clearing that hurdle, experienced PLENTY of erratic performance, may I suggest a rewording of the CDJ-400 manual:
"We at Pioneer created a USB hub that we don't feel confident will work in any way at all, which is why we make no guarantees of any sort. In fact, we advise you not to use the USB hub we provided."
11 November 2010 ADDENDUM - Played with MP3s burned to CD-ROMs (as data) today; this went extremely well. I also enjoyed success with an 8 GB thumb drive. So, not all that I hoped for, but still an improvement over my Numarks.