There was a great comment on HN a while back (I wish I remembered who said it) that said, "If you're asking about traction or revenue, you aren't making a seed investment".
Andy Bechtolsheim was a seed investor in Google -- he gave them a check based solely on their idea and who they were. Paul Graham/YC made a seed investment in reddit and Justin.tv and whole host of other companies -- none of them existed as more than idea when YC invested.
But VCs are risk averse people (ironic given they are in the risk business), so it makes sense that they would rather invest after a company can show a little traction, especially now that it's so easy to get that initial traction.
I just think we need a new term besides "seed" to differentiate it from the true seed investments.