There's 2 products (both with free versions) you can use for T4 intellisense:
One is by Clarius at http://www.visualt4.com/. But best to go with Tangible's T4 editor at http://t4-editor.tangible-engineering.com/T4-Editor-Visual-T4-Editing.html (available through the Extension Manager in VS2010 under tools).
Also if you want to get a bit more fancy with your templates, checkout http://t4toolbox.codeplex.com/.
Thanks to Pascal for the information here.