Format checking would be good to prevent mistakes. In my oppinion it should be 
configurable via a config parameter, if you lock on weeks or months. 

An alternative implementation for function weekstamp($timestamp) to lock months 
would be: 

  return strftime("%Y",$timestamp).strftime("%m",$timestamp);

together with having changed some text in language file from "week" 
to "timespan" and not using the add lock function from time registration page, 
this works well. 

I'll fix the validation problem for 1.0

Fixed, only valid week numbers are allowed now. Joachim: if you upgrade to 1.0
when it is released, and you still want to use the month lock instead of
weeklock, you can disable the check in the week_validate method in
modules/timereg/class.hours_lock.inc

Version cleanup.