Easiest answer first: There's no difference between your second two examples. By default, property accessors are atomic.
Atomic accessors in a non garbage collected environment (i.e. when using retain/release/autorelease) will use a lock to ensure that another thread doesn't interfere with the correct setting/getting of the value.
See the "Performance and Threading" section of Apple's Objective-C 2.0 documentation for some more information and for other considerations when creating multi-threaded apps.