Thanks for the test case.
It appears that 2.7 actually calls exec("execfile(filename)"),
when 3.1 directly calls exec(file_content).

The indirection seems necessary: the SyntaxError is detected by the pdb 
trace function; but this function has to run somehow...
With the patch below, pdb now runs exec("exec(file_content)").

I'm not sure how to write unit tests for pdb.
I don't know if it will be accepted for 3.1 final.

Index: Lib/pdb.py
===================================================================
--- Lib/pdb.py  (revision 73505)
+++ Lib/pdb.py  (working copy)
@@ -1211,7 +1211,7 @@
         self.mainpyfile = self.canonic(filename)
         self._user_requested_quit = 0
         with open(filename) as fp:
-            statement = fp.read()
+            statement = "exec(%r)" % (fp.read(),)
         self.run(statement)

 # Simplified interface