The latest iteration, Wilson says, revolves around early-stage Web services investing.

Now we’ve heard this concern before – including from Wilson, I think – as a brick in the “seed-stage bubble” argument. What interests me, however, is that there isn’t yet much of a track record of returns for investors to chase. Instead, they are chasing expected returns.

There obviously have been some big hits in the early-stage Web space – Groupon looks to be the latest, based on reports of a $2.5 billion acquisition by Google – but the vast majority of highly-valued Web services companies remain valued only by their VCs or secondary trading exchanges (where the buyers have virtually no knowledge of company financials).

Just take a look at Wilson’s firm, Union Square Ventures. According to recent fund performance data from UTIMCO, Union Square’s first fund has a whopping internal rate of return of 54.7%. When you look at cash-in/cash-out, on the other hand, you’ll notice that USV has not yet returned principal to its investors, let alone a positive return. Instead, most of the IRR is based on sky-high valuations for private, un-acquired companies like Twitter and Zynga.

This is no knock on USV. There is no reason why a 2004-vintage fund should have emptied out its portfolio yet, and there is speculation that it has generated some liquidity via later-round financings for some of its still-private portfolio companies.

Instead, it’s simply to point out that return chasers are banking on the anticipated rather than the demonstrated. To me, that feels even more problematic. At least those trying to copy real returns can get in early enough to catch the tail end of the boom… I fear this new breed won’t even get that early morsel before starving.