sqlaccess.dll is the in-database .NET data provider, System.Data.SqlServer. If you do any data access in .NET stored procedures, UDFs, triggers, UDTs, and UDAggregates, you are using this provider.
There is a version in Visual Studio Beta 1 (in \Program Files\Microsoft Visual Studio 8\Common7\IDE\PublicAssemblies) that (I think) Visual Studio uses in its Database projects. There’s a different version (the size is different by 1k) in \Microsoft SQL Server\MSSQL.1\MSSQL\binn.
I’ve renamed Visual Studio’s version to sqlaccess_vs.dll and copied SQL Server’s version to the VS PublicAssemblies directory. I’m not exactly sure what’s different yet, but am suspicious because the PublicAssemblies directory also contains ClrCppModule.dll. In SQLServer 2005 Beta 2, this piece is no longer needed to run CLR items in SQL Server.