This has been blogged about and promoted everywhere and it is true, it is a super cool feature that even the pre-.NET IDE supported, but VS2010 breaths new life into the feature with some great enhancements. For a very cool intro to the functionality take a look here where a member of the IDE team demonstrates the enhancements.
So, why on earth would I be blogging about “old” news. Well one thing that I found “missing” with this feature until now was the ability to use the keyboard to perform a block selection. Now with VS 2010 you can use Shift+Alt+[Up, Down, Left, Right] to perform a block selection. This is a great enhancement, and worth knowing about if you are a keyboard junkie.