Ah!  Intersting, I use the Task plugin as well... and yes I have been struggling to get a pretty date picker... I have not tried diabling the tasks plugin, but I noticed that some themes can muck it up too... for some reason the datepicker view is being extended in a bad way, ot overwritten... it would be good to have a fix for this (a nice tidy small picker below the User Block would be my desire).