Grant Thornton has prepare a paper entitled Market Structure is Causing the IPO Crisis .  Here is my understanding oftheir position.  The IPO market, especially for small IPOs started to decline before Sarbanes Oxley.  The Manning Rule and Order Handling Rule and decimalization decreased the margin for handling illiquid securities by brokerage houses.  Finally, online brokerage accounts have killed quality research and encouraged speculation.

The things that effect the IPO market are the cost of going public, the return for going public, the alternatives to going public, and the willingness of an investment bank to take you public (might be part of the cost).  While not stated explicitly in the report, they seem to imply that there is little incentive for investment banks to take small companies public or to create a market in their securities after the fact.  If true, I think this would contribute to the IPO downturn, but I do not think they have stated their case very strongly.