I'm getting a jagged edged looking line when I'm using the brush tool in PS CS5.  I've turned on smoothing, reduced the spacing to 1% and turned on anti-aliasing in the advanced settings.  My lines are smooth in Painter and Sketchbook Pro, but not PS.  Is there a setting I'm missing?

 I've attached an image of a quick line I did.  You can see that it's segmented into flat sections most obviously around sharper turns of the line.

Jagged line screenshot
 

Well, my advice is make your canvas bigger. Like, way bigger. This is the easiest way to get what you want.

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Yian 2012

Doesn't look like a resolution issue. What input device are you using? Tablet? Wacom? Cintiq? Other?

If a Wacom tablet, some people with a similar problem have found that changing from Standard Mode To Recognition Data solved it.

Recognition data should be the default mode. To double check and make sure that is the case, go to wacom property and double click on the tablet icon.

I read a post from CGTalk somewhere that standard mode has 100khz, and Recognition Data has 200 Khz. And with lower khz, the wacom picks up a lot less handshakes, thus gives out a smoother result. I gave it a try and switched to standard mode, and it does seem to have improved a bit. But the brushes become very unstable, and it is very frustrating.

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Yian 2012

If you use the 'Shift' key to assist in drawing, this will happen.  You know, holding down the Shift key to keep a straight line between clicks?  I've had things turn out this way doing that.

I'm using an Intuous4, so I'll try out that change to Recognition Data.

Yes!  Changing from standard to recognition data fixed it!   Thanks!! 

Good, it worked!