As a PhD student who starts his last year, I will soon start to look for post-docs. In this process, I will update my Curriculum Vitae. But as a title of this CV should I put "PhD in XXXXXX" or "PhD Student in XXXXXX" ?
Until you have your PhD officially, your CV should list "PhD Candidate" or "PhD Student," depending on your proper status. In your education section, you should list the month you expect to graduate (and you should say "(expected)" or similar next to the date).
(+1) to Chris Gregg's answer. In addition to that two points:
If you claimed to have a PhD (at the time of application) and you don't officially have the degree one can claim you are making a "false statement" and disbar you from the selection process.
[Assuming you want to finish your PhD first and then take up a job] You want your possible employer to consider you for a position after you finish your PhD. Saying you are a "PhD" already means you are ready for immediately employment. Furthermore you might want to have you employer know you might need a couple of days off for your viva, etc. Given that this might take place in the first couple of months of your employment and probably you are not allowed "vacation-time" yet, it is good to let them now early on.