I know exactly what you mean about the print, sign, scan cycle (I'm glad you added "recycle", though you might want to think about a shredder!) and it drives me a bit crazy too. Not only is it a waste of time, it also forces me to be in my office, where the scanner is located. If you've upgraded to Mac OS X 10.7 (Lion) it has the built-in capability of scanning in your signature, converting it to a special "signature" format, and letting you overlay that atop any PDF document, effectively "signing" it.

Problem is, there's no "Signature" menu, no "Signature" entry on any of the menus and there's no button atop the document you're viewing in Preview that has anything that appears to be useful in our endeavor.

There is, however, an "Annotations" toolbar and it turns out that's where you want to go. Once you're there, you can add signatures, insert a signature (as an annotation, totally makes sense) and even delete signatures as you improve or change things up.

Here's a typical document that requires a signature, in Preview:

To add a signature, I'll need to reveal the annotation toolbar. The easiest way to do that is to click on the pencil button just under the document name. I've highlighted it above.

The annotation bar is revealed:

There are a number of buttons worth learning about if you annotate PDFs, but for our purposes, all you need to focus on is the signature button, highlighted. Click on it and it'll generate a pop-up menu:

Since you have no saved signatures, you'll want to choose "Create Signature from Built-in iSight..." Do that and the built-in camera in your iMac or MacBook device will flip on and you'll see yourself.

At this point you need to grab a piece of white paper and a dark pen or marker and carefully sign the document, ideally with your signature on a straight line. Hold it up to the built-in camera and...

Move it back and forth until the entire signature fits into the highlighted area, then click on the "Accept" button.

Yes, it's tricky to do that and see what's going on both, let alone move the mouse to the right spot. I did that by moving the cursor over "Accept" before I held up the paper.

Once you accept the scan, rather surprisingly, nothing happens. The window just closes.

Upon closer examination, however, you notice that the cursor has turned into a cross-hair. That's what you want. Click on the spot in the PDF document where you want a signature and...

You can click and drag on any of the image "handles" to resize the signature to fit, and you can also click and drag from the middle of the image to move the signature so it's well positioned on the document:

Just a little bit of effort and the signature looks great! Better yet, Preview saves the signatures so next time it'll be way faster and easier...

Oh, and if you ever want to delete a signature or add another, go to the second option on that tiny pop-up menu: "Manage Signatures..." It looks like this:

That's all there is to adding a signature to a PDF via Preview. Very cool!