The International Association of Commercial Administrators collects state-by-state filings, including those of new business associations. I’ve found this data very valuable over the years in showing the competition among business forms. For some work using this data, see my article with Kobayashi, Choice of Form and Network Externalities.
The latest IACA data is out, reflecting formations through 2005. Here’s where you can find this and other reports.
The latest report shows that filings for LLCs are up in 2005 from the prior year in almost every state, while they are down for corporations in most states.