Will ASP.NET Membership handle a scenario with more than 2 million users? Does anybody have good or bad experiences with (very) large scale use of this provider?
Omar al Zabir has some recommendations: http://weblogs.asp.net/omarzabir/archive/2009/03/14/optimize-asp-net-membership-stored-procedures-for-greater-speed-and-scalability.aspx
Omar Al Zabir also posted 10 ASP.NET Performance and Scalability Secrets with one of the items being How to query ASP.NET 2.0 Membership tables without bringing down the site. Basically it's to do with querying the database in a way that makes sure the correct indexes are being used. There is also another tip about Profiles in this list. I'm not sure if these tips are the same as the ones in the article recommended by Matt.
I'm fairly sure Omar's http://www.pageflakes.com/ site uses the SQLMembership ASP.NET Membership provider and he would probably be a great person to speak to to make a judgment call on whether this scenario is feasible. Given his community efforts I'm sure you could track him down via the sites Contact page. Get him to post the answers here!