I think that one of the most useful metrics when looking at public valuations is the P/E to growth ratio. It just makes so much sense that faster growing companies would have higher P/E's and the contrary, as we see in recessionary times, is just as true.
Over the years, I have seen this ratio, on a normalized basis, approximate 1.2. As an illustration, if a company is projecting 20% growth, you would think the market would reward the firm with a P/E of around 24x.
The latest B of A Internet report has a some nice data (p 12-13) that looks at their coverage universe, segmented by large and mid-cap and advertising and commerce that shows which of the covered firms would be above the valuation line (more than fully valued), or below the line (less than fully valued). From the charts, it seems as if advertising is about fairly valued today, and with a limited universe, e-commerce is not significant enough to note.
From a macro industry view, unless we see a continued deterioration of expectations, it seems as if we have achieved a reasonable balance between valuation and growth.