MATLAB 7.8.0 (R2009a) - 168 163 162 characters
Starting from Jacob's answer and inspired by gwell's use of any non-whitespace character to fill the arc, I managed the following solution:
[w x r]=strread(input('','s'),'%d-%d%d');
l='o -/|\-/|\-';
X=meshgrid(-r:r);
T=atan2(-X',X)*180/pi;
T=T+(T<=-~w)*360;
T(T>x|T
And some test output:
>> arc
0-135 4
\||||////
\|||///-
\||//--
\|/---
o----
I could reduce it further to 156 characters by removing the call to disp, but this would add an extra ans = preceding the output (which might violate the output formatting rules).
Even still, I feel like there are some ways to reduce this further. ;)
