My attempt in C (70 chars):
P(char*s){char*e=s+strlen(s)-1;while(s=e;}
[Edit] Now actually working
[Edit 2] Reduced from 74 to 70 by using default int return
In response to some of the comments: I'm not sure if that preprocessor abuse counts - you could just define the whole thing on the command line and make the function one character.
