It seems that PyDev only triggers break points from the main thread. Break points in background threads seem to be ignored. Here's some example code:
print 'count is %d.' % count # set break point here
print 'calling from main thread:'
print 'calling from bg thread:'
raw_input('press enter to quit.')
That breakpoint is hit with count 13, but not 23. This Stack Overflow question has a bit more detail and some work arounds: