It's always surprised me that, given the number of artists Apple targets with their offerings, they have absolutely no offering like this that artists can use.

Here's a review of a similar workflow from 2002 by the Penny Arcade guys using a Windows tablet.

http://www.penny-arcade.com/2002/11/27/

and here is their take on the desire of web artists to have a tablet from Apple right before the iPad came out.

http://www.penny-arcade.com/comic/2010/1/22/