Venus had grossly different initial conditions -- starting with a different composition -- and never had an atmosphere resembling ours. It's geology today, to take a concrete example of the differences, lacks plate tectonics. The atmosphere of Venus is 90 times as massive as ours. It isn't at all a good model for terrestrial greenhouse warming.
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I'm finding myself on a bit of a campaign to damp down exaggerated catastrophe scenarios. The leap from "huge negative change" to "End of the World" is taken far too easily in discussions in this end of the blogosphere. I propose that we regard climate changes that fall outside the range seen in the last 100 million years as not credible. Besides, cooling the Earth is cheap and easy (only limited guarantees on side-effects, though).

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