From what i can decipher from the site, the main difference is support options.
ed Hat Professional Workstation is designed for the advanced user requiring a single Linux desktop deployment with limited support and management capabilities. Customers looking for advanced support options and server features or planning on deploying Linux on a large scale should look to Red Hat Enterprise Linux for additional options.
Features not available with Red Hat Professional Workstation (but standard in Red Hat Enterprise Linux) include:
* Additional Support options -- Customers needing support beyond basic installation should look to Red Hat Enterprise Linux.
* Open Source Architecture Integration -- Red Hat Enterprise Linux provides the necessary components and support for integration into a managed corporate desktop infrastructure. Red Hat Enterprise Linux will be the core platform of the Open Source Architecture.
seems like its mainly the supports options offered.
Added: Just noticed on the comparison chart for enterprise, that WS does not supports dual proccesors, but professional workstation does.