The root of this bug is that lxtask is miscounting the number of CPUs in xfce-taskmanager-linux.c (in get_system_status() at line 355), it is making an assumption that for each processor that there will be a line starting with the case sensitive string 'processor' but on Rasbian the architecture-dependent /proc/cpuinfo only has a line starting with 'Processor'.
To work around this, assume that the number of processors will always be at least one, as the attached patch 02-numcpu.patch does.
This illustrates the underlying difference in data between an i386 kernel and the arm6l kernel's procinfo entries related to processor:
pi@raspberrypi ~/lxtask-0.1.4 $ uname -a
Linux raspberrypi 3.6.11+ #371 PREEMPT Thu Feb 7 16:31:35 GMT 2013 armv6l GNU/Linux
pi@raspberrypi ~/lxtask-0.1.4 $ grep -i processor /proc/cpuinfo
Processor : ARMv6-compatible processor rev 7 (v6l)
[rbulling@foobar ~]$ uname -a
Linux foobar.example.com 2.6.18-274.7.1.el5xen #1 SMP Mon Oct 17 12:14:02 EDT 2011 i686 i686 i386 GNU/Linux
[rbulling@foobar ~]$ grep -i processor /proc/cpuinfo
processor : 0
processor : 1
processor : 2