This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Each portal vendor provides proprietary APIs that can help you do things like customization on extra personalization. But if you stick to the standard, JSR168 or JSR286, then your portlets are cross platform, and there won't be a difference.