I cleared WSS 3.0 (70-541) Microsoft Certification exam recently. With this, I got the title "Microsoft Certified Technology Specilaist (MCTS)). There are many topics related to development covered and preparing for the exam will certainly take your knowledge to the next level. Having experience in WSS and MOSS certainly helped me to get through.
Here is the brief list of topics covered in detail.
* Lists / Documents
* SharePoint Object Model (SPSite/SPWeb etc)
* Working with Workflow
* SharePoint Features
* Deplying sites
* Site Definitions, List Definitions, ONET.XML
Along with the experience in WSS and MOSS development, I primarily used Inside Windows SharePoint Services 3.0 book which gives a very good insight of WSS 3.0