In the past year or so, a few installations had begun to experience problems with the SQL Server 2005 security cache (aka TokenAndPermUserStore) growing too large over time. Some manifestations are connection and query timeouts and queries that take a long time.
The folks at PSS published the canonical blog entry about this problem, including knowledge base articles and suggestions for SQL Server 2005, and also mention of the name of the SQL Server 2008 parameters to configure the size of this cache at the end of the article.
In SQL Server 2008 the size of TokenAndPermUserStore is configurable as mentioned in the SQL Server 2008 Books Online. I happened across it yesterday trying in researching a user inquiry. The configuration options, access check cache quota and access check cache bucket count, are mentioned in the books online without much fanfare or much description. But a knowledge base article, (KB955644) published the day that SQL Server 2008 shipped, describes the defaults and suggest some guidelines for configuring these parameters. Excellent.