Is There a YCombinator Valuation Bubble or Not?

At the end of Friday's piece, I asked you guys to vote on SurveyMonkey what I should write about this weekend. Over 50% of you said you wanted me to discuss the buzz around the bubble in valuations for Y Combinator companies. ( Full results: http://jc.is/HE3AV5 )
I speak with six to 12 investors a week, both angels and VCs, as well as a half-dozen founders. Almost every discussion for the past couple of weeks has shifted to the valuations of the latest YC crop.
[ Note: In this piece I will not reference any specific knowledge I have as an angel investor. Everything here is based on public discussion of these issues. ]
For background, the valuation of a startup is the number investors and founders come up with as the full monetary value of a business during a financing event.
Valuations are always a hot topic, but they are delicate for two specific reasons: