Does anyone know of a plugin that will allow me to create a registration/ membership form on my blog which can send data to my salesforce account, as well as update a page on my wordpress site with the data that gets submitted?
That’s not possible on a WordPress.COM hosted blog. You need to hire a web host http://wordpress.org/hosting and set up your own WordPress.org software install.
WordPress.COM and WordPress.ORG are completely separate and have different logins, features, run different versions of some themes with the same names, and have separate support forums. Read the differences here http://en.support.wordpress.com/com-vs-org/
WordPress.org offers free software that you can install on a web server. You can upload and install themes and plugins, run ads, conduct ecommerce and edit the database. Learn about the differences here. http://support.wordpress.com/com-vs-org/
If you don’t have a username account at WordPress.ORG click http://wordpress.org/support/ and register one on the top right hand corner of the page that opens, so you can post to the support forums there.