We inadvertently went to WDW during that week this year (didn't realize it when we booked it). When we realized it we rearranged our schedule so that we weren't in the parks at all on that Monday (as advised by our friends here at the Dis). But we spent the Saturday before that at AK and the Sunday before it at SeaWorld and had no problems, crowd-wise. AK had fairly short lines (DD rode Kali River Rapids twice in a row with almost no wait). The rest of the week was just fine. Bit larger crowds than our previous January visit, but that was just 4 months after 9/11, so obviously the crowds were very low then.
THere were a few soccer tournaments in town and I was told by folks here at the Dis that most of those sorts of events have Monday off, so that is why the parks would be especially crowded that one day. But I certainly would not worry about it too much........P