Hope I can help here.....I went looking for the Wilton info. Here is the URL:
http://www.wilton.com/decorating/basic/brush.cfm
Also, I've seen pics and read descriptions of it being done with both Buttercream and Royal icings. Anything past that is up to you. Use your imagination.
One tip I got from my Wilton instructor is that you use the following paint brushes as listed:
Bevel for shading
Sq tip for regular embroidery
Round for free hand designs.
I picked up all these brushes, but haven't had a cake to use them on yet.
Hopefully, my reply will bump you so that others, who have done the brushed embroidery, can help.
Good luck! 
Royal icing or a crusting buttercream can be used for brush embroidery. To try it out, place a small simple flower patter under a piece of waxed paper, so you can trace the design. Here is a tutorial on the Wilton site.
http://www.wilton.com/decorating/basic/brush.cfm
Oops, I see someone got there first.
The variety of brushes may help, but I had good success with a little soft brush
lol, JoAnnB .....it never hurts to get the same info from more than 1 location.
And the Wilton site has a lot more info....here's the URL for the Wilton site. It has a lot for you to explore.
http://www.wilton.com/decorating/basic/index.cfm
I even found the painting info page which tells you which brush should be used for which kind of technique 
I have done 3 cakes with brushed emb. one was on fondant and 2 on crusting BC. Take about 1c of bc icing add 1/2 tsp piping gel. I mark my cake with a fondant flower cutter using tip #2 trace the mark on the inside line use a small brush dipped in water removing the excess and brush the design inword to the middle not to much going around the whole flower. I have pics in my photos. Its pretty easy to do but the more you practice it looks nicer. The first pic is my latest one , there is another further down the page 2 tier with pillers and another on the next pg all white which was my first one.
Thanks everyone...I did have another question about it though. How do you get it to look really sparkly or shiny? Can this be done with piping gel alone instead of BC or royal icing? Can you add luster dust to the BC or royal or piping gel? The reason why I ask is I was at Colette Peters website and also the confetti cakees website. I believe it was the Confetti Cakes where I saw the sorta glittery brushed embroidery effect.
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