This question deals with a third-party plugin, but there may be enough crossover with the stock jquery UI datepicker that someone might be able to help out.
1) Set a maxDate of 0 or new Date() is set in the datetimepicker options
$('.foo').datetimepicker({maxDate:0})
2) Press "now" button
!3) Now button does not set time to the current time, but the time when the page was loaded
Example: http://jsfiddle.net/rf3gk/
How can I work around this? Is there a fix I can do, either in an external file or in the addon itself?