I found one workaround:
Use ‘Edit scheme’ under the ‘Product’ menu, select ‘Run’ in the left bar, the ‘Info’ tab, and change the Debugger to gdb (this does not apply to xcode 5, which no longer has gdb).
Apparently Apple thought they’d fixed this bug in xcode 4.3.1, but it still happens. I submitted some extra debug information they requested, so I’m hoping it’ll be fixed for the next release of xcode. It’s still failing in 4.3.2. See https://devforums.apple.com/message/623694 for an update from Apple.
UPDATE
I’ve tried various cases I was having trouble with, and they all seem to be working fine with lldb in Xcode 4.4.1 – hence I highly recommend upgrading if you’re having this problem.