What you want is wx.SingleInstanceChecker.
Ah, you mean that with the great documentation:
--- snip ---
Package wx :: Class SingleInstanceChecker
Type SingleInstanceChecker
object --+
>
SingleInstanceChecker
···
--------------------------------------------------------------------------------
Proxy of C++ SingleInstanceChecker class
--------------------------------------------------------------------------------
Method Summary
SingleInstanceChecker __init__(self, name, path)
__del__(self)
__repr__(self)
bool Create(self, name, path)
bool IsAnotherRunning(self)
--- snip ---
Ok, there's http://wxwidgets.org/manuals/2.5.3/wx_wxsingleinstancechecker.html
Seems one can use it for an own 'singular' app, but cannot check if an other app is running (e.g. before my app would start it) - perhaps enough for the OP, but not enough for me. 
Best regards,
Henning Hraban Ramm
Südkurier Medienhaus / MediaPro
Support/Admin/Development Dept.