"Hey, I like your idea so much that I actually want to execute on it. Sorry."
At least Z's guilt can dissipate if it turns into a success.
Personally, I've seen people use my ideas and thoughts. Their error is not to include me.
Blog Nation I think could've started on the back of my prototype site (I'm not totally sure), but it has now crashed and burned, and I emailed the founder with a link before BN started. At first I was somewhat peeved, and now it's somewhat amusing.
People with ideas and executive ability are rare perhaps. Some people can spot a good idea but not generate one, and they are always on the lookout for ideas. Others generate but can't separate the wheat from the chaff.
For the former, one might want all the credit and glory and leave the latter in the cold and isolated as much as possible. Both need to reconcile with each other as best as possble, as is happening now in the Facebook saga.
Greenspan has learned the hard way perhaps. Sometimes you have to know when to move on.