Due to our plan to deliver item centric PDM capabilities to SolidWorks Enterprise PDM 2009 in SP2, we will not deliver SP1, but will combine the software corrections from SP1 and SP2, along with the new features, into 2009 SP2. The new item centric PDM capabilities will provide entirely new ways to create complete Bill of Materials and communicate with downstream applications such as PLM and ERP systems.Our plan is that all future Service Packs will continue to align with the corresponding SolidWorks Service Pack release, such that SolidWorks Enterprise PDM 2009 SP3, SP4, SP5 will align with SolidWorks 2009 SP3, SP4, SP5.

Source: www.SolidWorks.com