In an unannounced update, individual contact pages in the People Hub now displays users' Twitter bio as a notes field. This hasn't come about by any firmware update, and so must be a backend change on the Microsoft Live servers that act as the intermediary between Twitter and your Windows Phone 7 device. It's uncertain as to when this update was implemented, but it has just been noticed by WPCentral.
It would be helpful to see similar updates taken from Facebook and LinkedIn for contacts who don't have a Twitter account. Having said that, this is great for quickly reading up on a contact you're not familiar with - which before the social networking age would have been a contradiction in terms!
If you already have notes for a contact from their entry in your Google or Live address book, don't worry they won't be overwritten. Instead, you'll find two notes sections.