Already have one in line, it's even better than Brushes. It's called ArtStudio:
FEATURES:
- 25 brushes including pencils, brushes, eraser, smudge tool, bucket fill (simple and smart mode), glow, tube, airbrush and many more
- advanced settings offer 8 different parameters and real-time preview for all brushes
- simulated brush pressure
- line smoothing, antialiasing, zooming with filtering
- 3 drawing modes: free draw with smoothing, free draw without smoothing, lines
- very comfortable interface; screens can be displayed in simple or advanced mode.
- quality and performance unmatched by any other app in the app store
- 5 layers with options: add new / delete / reorder / duplicate / merge / clear / transparency / visibility
- multi-touch navigation with unlimited zoom
- advanced color editing with customizable color swatches
- 6 levels of undo / redo
- drawing lessons (animals, human, 3d, perspective)
- filters: blur, sharpen, edge detect, sepia and more...
- load/save/export to iphone gallery / import from gallery / get from camera / send by e-mail
- additional "shake menu" with the most frequently used functions