I was about to send the patch with exit 1 commented out, then I decided just to write on my Jira comment that you can get rid of it.
It's difficult to know if they will solve the bug before 0.21, although the Debian netbase maintainer already admits that it won't be trivial: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=560238#154
OTOH, sooner or later it will be fixed (and, on the positive side, Ubuntu 10.04 still relies on netbase 4.35).
I tried to reproduce what I saw in your transcript (my environment: debian testing, netbase 4.40, 2.6.32-trunk-amd64).
Clients work, DFS interface and file preview works, example job fails:
10/01/27 13:34:14 WARN mapred.JobClient: Error reading task output http:10/01/27 13:34:14 WARN mapred.JobClient: Error reading task output http:10/01/27 13:34:29 INFO mapred.JobClient: Task Id : attempt_201001271246_0002_r_000031_0, Status : FAILED
java.io.IOException: Task process exit with nonzero status of 1.
That's why I think it would be a waste of time in some cases: you wait till the NN exits safe mode, launch your job... and it fails unexpectedly.