After I had a quick look at the feasibility of this feature I spend some time on implementing it for Xdebug's HEAD branch that is going to become Xdebug 2.1. Variable modification tracing can be enabled by setting the php.ini xdebug.collect_assignments setting to 1. Of course this can also be done in either .htaccess or by using ini_set(). This setting requires general execution tracing to be enabled as well and it's only available for human readable trace files (the default format).
He gives examples of how two example scripts would be logged to the trace file - one inside of a basic function and another showing updates to the properties on a class.