Hi!
It possible to have sketch effect in main camera like:
THANKS
try hiring texel on the irc channel to do it for you.
At a guess I’d say its quite complex 
You can try Edge Detect Image Effect:
Scene without edge detection
Detected edges of the same scene
From Unity documentation :
"Edge Detect Effect Normals
This Edge Detect image effect creates outlines around edges. Note that this version of the image effect actually works on real geometry i.e. scene depth and normals to detect edges.
As with the other image effects, this effect is only available in Unity Pro. Make sure to have the Pro Standard Assets installed.
Properties
Mode Chose between Thin and Thick. Thick will take more samples into consideration to construct outlines.
Edge Sensitivity
Depth The depth sensitivity that defines the needed depth difference when identifying edges.
Normals The normals sensitivity that defines the needed difference between neighbouring normals when identifying edges.
Background options
Edges only Blend the background with a fixed color.
Background The color used when Edges only is > 0.
Hardware support
This effect requires a graphics card with pixel shaders (2.0) or OpenGL ES 2.0. Additionally, depth texture support is required. PC: NVIDIA cards since 2004 (GeForce 6), AMD cards since 2004 (Radeon 9500), Intel cards since 2006 (GMA X3000); Mobile: OpenGL ES 2.0 with depth texture support; Consoles: Xbox 360, PS3.
All image effects automatically disable themselves when they can not run on end-users graphics card."
http://unity3d.com/support/documentation/Components/script-EdgeDetectEffectNormals.html
Note that you need to have Unity iOS Pro license to be able to use that.
I havevn’t try it myself, but from the sound of it, it sounds like it could work. I know iPhone 3GS+ supports OpenGL ES 2.0, and “depth texture” is supported on iOS (since iOS 4.1, GL_OES_depth_texture is supported). Although there is another Unity documentation which seems to indicate this might not work. It seems contradictory so you just have to try it out 
"Using Depth Textures
iOS
This feature is not supported for iOS targets."
The question I have is, why is Depth Textures not supported on Unity iOS when clearly GL_OES_depth_texture is supported since iOS 4.1 ?? UT any answer?
Thank you for reply… Edge Detect works only with 3d object and not with Texture…
Actually, I was wrong. I referenced the wrong image effect - the previous one detect geometry edge (edge normal), this one works on the color difference which is probably closer to what you want.
This one should work :
Edge Detection image effect
Edge Detect image effect adds black edges to the image where color differences exceed some threshold.
If proper geometry based edge detection is needed, the Standard Assets also provide a more complex normal-based edge detection.
Like all image effects, Edge Detection is available in Unity Pro only. Make sure to have the Pro Standard Assets installed.
Edge Detect image effect applied to the scene
Threshold Edges will be displayed whenever color difference in neighboring pixels exceeds this amount. Increase this value to make edges less sensitive to texture or lighting changes.
Hardware support
This effect requires a graphics card with pixel shaders (2.0) or OpenGL ES 2.0. PC: NVIDIA cards since 2003 (GeForce FX), AMD cards since 2004 (Radeon 9500), Intel cards since 2005 (GMA 900); Mobile: OpenGL ES 2.0; Consoles: Xbox 360, PS3.
All image effects automatically disable themselves when they can not run on end-users graphics card.
http://unity3d.com/support/documentation/Components/script-EdgeDetectEffect.html
It doesn’t look like it requires Depth Texture support (which currently is unsupported on mobile platform), so all it need is OpenGL ES 2.0 support - which iPhone 3GS+ should be able to handle.