How was this done? It seems like a varying thickness brush effect, not sure. Can anyone duplicate this style and provide instructions? Thanks!

The picture below is cropped, please see full image here: http://jimsturgessonline.com/fanart/albums/userpics/10001/HOT___NASTY.jpg

That was done in a raster program.
The face is pixelated, it doesn't seems it was vector.
Some layer filtering going on with the lines, as they have a white edge on the smudged background.
To me it seems the lines that you are looking for are the result of a scanned paper image or such.
Judged by the look it would take more than 20000 nodes to reproduce that image in vectors.

To create a blot effect to a simple path, there are some options:
drawing a blot-draw a circle, convert it to path by pressing Ctrl+C, set a fill and no outlines.
add some new nodes to it by the node editor, or with the extension for that
Add some jittering to the path by the jitter extension
or
fractalize the path, and use simplify path on it

Then, to draw such lines, draw a base path you want to "blotify", and
run the pattern along path extension with such a blot for the pattern.
After that, you can edit the nodes a bit, add new ones, apply jitter/fractalize, simplify path, and
there is the tweak tool - - wich you can call by Shift+F2, with you can make parts of the line shapes thinner.

You may be able to create a similar effect in inkscape using the Awesome Vector Splatters that chrisdesign made using real inkblots then scanning them in and using inkscape/gimp to turn them into some Awesome resources for us all to enjoy!

I think you will really like this:

http://chrisdesign.deviantart.com/art/20-Vector-Splatters-SVG-305714052

And this:

http://chrisdesign.deviantart.com/art/33-Vector-Paint-drips-312735558

And this:

http://chrisdesign.deviantart.com/art/14-Vector-Paint-drips-long-312776986

Not to mention the many other awesome svg files he offers. And if you use Gimp, you will find a ton of resources for The Gimp as well.

Chris has a blog where he does some tutorials and shows off his awesomeness as well at:

http://chrisdesign.wordpress.com/

http://DeviantArt.com is an awesome place to find some really cool filters for inkscape as well.
And don't forget to check out http://OpenClipArt.org which has tons of really nice SVGs for
us all to enjoy. Chris also shares some really nice stuff there as well.

And if you want some really Awesome Inkscape Filters, I would highly recommend to check out:

http://openclipart.org/user-detail/ivan_louette

Ivan has some amazing filters!