The apparent culprit is the file diff.pro (the PostScript Prologue file for a2ps), which is usually located at /usr/share/a2ps/ps/diff.pro. You can replace your diff.pro file with my modified diff.pro.
The modificarions made are (unified diff):
--- diff.pro-orig 2010-08-14 12:35:00.320960289 +0200
+++ diff.pro 2010-08-14 13:07:18.227961317 +0200
@@ -2,7 +2,7 @@
% PostScript Prologue for a2ps (meant for diffs)
%
% $Id: diff.pro,v 1.2 1998/04/08 14:54:21 demaille Exp $
-%
+% jj: modified to fix pdiff 2010-August-14
%
% This file is part of a2ps.
@@ -68,11 +68,12 @@
Show
} bind def
+%% jj:
% Removed
/k {
0 0 0 FG
- false BG
- true UL
+ 0.8 0.8 0.8 true BG
+ false UL
false BX
%Face: Keyword Courier bfs
Show
@@ -83,7 +84,7 @@
0 0 0 FG
0.9 0.9 0.9 true BG
false UL
- false BX
+ true BX
%Face: Keyword_strong Courier bfs
Show
} bind def