The first week of Jan 2006 was very crowded. We were there then as TX schools were out that entire week. I think many schools in CA were also out that week. We are not out that entire week for 2007, but I don't know about CA schools. I think the New Years week is typically lower attendance and that 2006 was just unusual because some schools were out.
If you do go then, be sure to catch all of the XMAS events/shows that will probably only be going on the first few days. The XMAS decorations started coming down during the week, and Small World shut down for removal of the XMAS overlay, but we were able to ride it early in our trip.
Don't know how early you can make reservations. We were fortunate enough to get the great AP rate of $89 for DLH that we booked in early November!