In case you haven't seen the announcement on the Google Data API blog, there is a new API available for Google Finance Portfolios.

The Google Finance API makes it easy to create and access investment data for your users -- ranging from updating their investment holdings and watchlists to retrieving current portfolio value and performance. With your applications, users can monitor their portfolios and transactions and keep positions up-to-date and in-sync. If you have a great idea for a portfolio application, give the new API a try!