I read some conjecture on a previous forum (but I couldn't find it since there's no search function) about whether the upcoming Model S cars would contain proximity sensors and whether the current ones were pre-wired for it. Apparently, the answer is no. Here's an excerpt of an email from firstname.lastname@example.org:
Parking sensors are not part of current Model S and are not being planned for the near future. However, we are evaluating the option for future models.
There are currently no provisions for parking sensors in Model S, so retrofitting would be quite involved, and would not be integrated with the other features of Model S. No warranties would be voided unless significant dissembly and modifications to parts is required to install an aftermarket parking sensor system. It is hard to recommend any options as none will integrate cleanly with Model S, but other owners on the forums have explored various solutions and some might be to your liking.