A colleague from work recommended me a Painting Tool for my iPhone called "brushes". The last few weeks I really got into it. I bought a "pogo stylus" for my phone and started doodling around with "brushes". There are several painting tools for the iPhone available but this is definitely the best so far.
The iPhone display is not pressure sensitive so you have to set the opacity manually but that doesn't bother me at all. Brushes provides multiple undo and redos, a pop up eydropper tool and many more cool features.

These are Paintings I've done on my iPhone with the stylus.
I'm really enjoying this tool. It feels like a portable cintiq!!
Hope you like my lil' doodles...

You can get it in the Apple AppStore for around 5bugs but right now it's on sale for 0.99cents....so go and get it!!!