Comment 14 for bug 507389

Incidentally, the excellent link you mentioned at does illustrate the shortcomings of snapshot serialization using an example that is quite similar to the issue at hand: a /write skew/ anomaly in stock reservation could allow the real stock level to become negative when it really wasn't meant to be.
And one of the workarounds for /write skew/ mentioned in the article is Promotion, which is what SELECT FOR UPDATE really does :-)