The correct answer should be "it depends". As already stated, there are no guarantees even it seem to work in some cases, but with larger tables, parallell plans, many users and enterprise edition (advanced read ahead) you can't rely on the order of the result.
ALWAYS use the ORDER BY clause if you depend on the order.
/Håkan Winther
MCITP:Database Developer 2008
MCTS: SQL Server 2008, Implementation and Maintenance
MCSE: Data Platform