XBee shield communication
The only way I can think of accomplishing this without some sort of proximity sensor is to measure the signal strength of the XBees.

From page 41 of the datasheet:


  DB (Received Signal Strength) Command
  
  AT Command: ATDB
  
  Parameter Range [read-only]:  0x17-0x5C (XBee), 0x24-0x64 (XBee-PRO)
  
   DB parameter is used to read the received signal
  strength (in dBm) of the last RF packet received. Reported values are
  accurate between -40 dBm and the RF module's receiver sensitivity.
  Absolute values are reported. For example: 0x58 = -88 dBm (decimal).
  If no packets have been received (since last reset, power cycle or
  sleep event), “0” will be reported.


You can use this as a rough estimate of the distance between two sensors. You will have to correlate signal strength to actual distance and obstructions and other factors will cause the reported value to be off.

How to measure XBee signal strength?

This page has an example using Processing (on the computer): XBee Signal Strength