SQL Server 2000 had a hard-coded default of 255 max worker threads.  This setting was usually enough for a typical out-of-the-box OLTP application. 

SQL Server 2005, however, has gotten much smarter.  It will establish the max worker thread default based upon the machine architecture (32 vs 64bit) and/or number of processors. The value will be set based on a table that is described in the SQL Server BOL in the section 'max worker threads Option'.

I hope this helps,

-Kevin