Security System (using wireless signals).
No gesture recognition like WiSee or Wi-VI see thru walls.
WiSee http://wisee.cs.washington.edu/
Wi-Vi http://people.csail.mit.edu/fadel/wivi/index.html
Just start with motion detection
I think at Olimex you have most, if not all of the hardware
for detection and reaction.
A hard part of the project... Software and only using Open
Source Software for development.
Maybe start with code from:
GnuRadio http://gnuradio.org/redmine/projects/gnuradio/wiki
(Signal Capture)
Spyder http://pythonhosted.org/spyder/index.html
(Signal Processing-SciPy)
(Signal Plotting-matplotlib)
And the hardest part of the project, is trying to find experts
in Python Programming :-(